Hard drive slow down suddenly

My hard drive has become very very slow and I don't know why. You can see the performance here:

sudo hdparm -t /dev/sda

/dev/sda:
 Timing buffered disk reads: read(2097152) returned 28672 bytes

How can I diagnostic my Hard Drive and fix it?

I don't lose any data. The Hard Drive is just became super slow and I don't know why ;(

EDIT:

You can also see the output of this command:

sudo smartctl -a /dev/sda

Here:

smartctl 6.5 2016-01-24 r4214 [x86_64-linux-4.4.0-36-generic] (local build)
Copyright (C) 2002-16, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF INFORMATION SECTION ===
Model Family:     HGST Travelstar 7K1000
Device Model:     HGST HTS721010A9E630
Serial Number:    JG40006PGKHV1C
LU WWN Device Id: 5 000cca 6acc7f5c5
Firmware Version: JB0OA3B0
User Capacity:    1 000 204 886 016 bytes [1,00 TB]
Sector Sizes:     512 bytes logical, 4096 bytes physical
Rotation Rate:    7200 rpm
Form Factor:      2.5 inches
Device is:        In smartctl database [for details use: -P show]
ATA Version is:   ATA8-ACS T13/1699-D revision 6
SATA Version is:  SATA 2.6, 6.0 Gb/s (current: 6.0 Gb/s)
Local Time is:    Sun Jan  1 14:32:50 2017 CET
SMART support is: Available - device has SMART capability.
SMART support is: Enabled

=== START OF READ SMART DATA SECTION ===
SMART overall-health self-assessment test result: PASSED

General SMART Values:
Offline data collection status:  (0x00) Offline data collection activity
                    was never started.
                    Auto Offline Data Collection: Disabled.
Self-test execution status:      ( 117) The previous self-test completed having
                    the read element of the test failed.
Total time to complete Offline 
data collection:        (   45) seconds.
Offline data collection
capabilities:            (0x5b) SMART execute Offline immediate.
                    Auto Offline data collection on/off support.
                    Suspend Offline collection upon new
                    command.
                    Offline surface scan supported.
                    Self-test supported.
                    No Conveyance Self-test supported.
                    Selective Self-test supported.
SMART capabilities:            (0x0003) Saves SMART data before entering
                    power-saving mode.
                    Supports SMART auto save timer.
Error logging capability:        (0x01) Error logging supported.
                    General Purpose Logging supported.
Short self-test routine 
recommended polling time:    (   2) minutes.
Extended self-test routine
recommended polling time:    ( 174) minutes.
SCT capabilities:          (0x003d) SCT Status supported.
                    SCT Error Recovery Control supported.
                    SCT Feature Control supported.
                    SCT Data Table supported.

SMART Attributes Data Structure revision number: 16
Vendor Specific SMART Attributes with Thresholds:
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  1 Raw_Read_Error_Rate     0x000b   100   100   062    Pre-fail  Always       -       0
  2 Throughput_Performance  0x0005   100   100   040    Pre-fail  Offline      -       0
  3 Spin_Up_Time            0x0007   178   178   033    Pre-fail  Always       -       2
  4 Start_Stop_Count        0x0012   099   099   000    Old_age   Always       -       1788
  5 Reallocated_Sector_Ct   0x0033   100   100   005    Pre-fail  Always       -       0
  7 Seek_Error_Rate         0x000b   100   100   067    Pre-fail  Always       -       0
  8 Seek_Time_Performance   0x0005   100   100   040    Pre-fail  Offline      -       0
  9 Power_On_Hours          0x0012   082   082   000    Old_age   Always       -       8005
 10 Spin_Retry_Count        0x0013   100   100   060    Pre-fail  Always       -       0
 12 Power_Cycle_Count       0x0032   099   099   000    Old_age   Always       -       1765
191 G-Sense_Error_Rate      0x000a   100   100   000    Old_age   Always       -       0
192 Power-Off_Retract_Count 0x0032   100   100   000    Old_age   Always       -       69
193 Load_Cycle_Count        0x0012   098   098   000    Old_age   Always       -       26038
194 Temperature_Celsius     0x0002   181   181   000    Old_age   Always       -       33 (Min/Max 12/50)
196 Reallocated_Event_Count 0x0032   100   100   000    Old_age   Always       -       2
197 Current_Pending_Sector  0x0022   100   100   000    Old_age   Always       -       0
198 Offline_Uncorrectable   0x0008   100   100   000    Old_age   Offline      -       0
199 UDMA_CRC_Error_Count    0x000a   200   200   000    Old_age   Always       -       1
223 Load_Retry_Count        0x000a   100   100   000    Old_age   Always       -       0

SMART Error Log Version: 1
Warning: ATA error count 65535 inconsistent with error log pointer 2

ATA Error Count: 65535 (device log contains only the most recent five errors)
    CR = Command Register [HEX]
    FR = Features Register [HEX]
    SC = Sector Count Register [HEX]
    SN = Sector Number Register [HEX]
    CL = Cylinder Low Register [HEX]
    CH = Cylinder High Register [HEX]
    DH = Device/Head Register [HEX]
    DC = Device Command Register [HEX]
    ER = Error register [HEX]
    ST = Status register [HEX]
Powered_Up_Time is measured from power on, and printed as
DDd+hh:mm:SS.sss where DD=days, hh=hours, mm=minutes,
SS=sec, and sss=millisec. It "wraps" after 49.710 days.

Error 65535 occurred at disk power-on lifetime: 8005 hours (333 days + 13 hours)
  When the command that caused the error occurred, the device was active or idle.

  After command completion occurred, registers were:
  ER ST SC SN CL CH DH
  -- -- -- -- -- -- --
  40 51 08 38 00 00 00  Error: UNC at LBA = 0x00000038 = 56

  Commands leading to the command that caused the error were:
  CR FR SC SN CL CH DH DC   Powered_Up_Time  Command/Feature_Name
  -- -- -- -- -- -- -- --  ----------------  --------------------
  60 08 30 38 00 00 40 00      00:03:34.637  READ FPDMA QUEUED
  60 08 28 30 00 00 40 00      00:03:34.637  READ FPDMA QUEUED
  60 08 20 28 00 00 40 00      00:03:34.636  READ FPDMA QUEUED
  60 08 18 20 00 00 40 00      00:03:34.636  READ FPDMA QUEUED
  60 08 10 18 00 00 40 00      00:03:34.636  READ FPDMA QUEUED

Error 65534 occurred at disk power-on lifetime: 8005 hours (333 days + 13 hours)
  When the command that caused the error occurred, the device was active or idle.

  After command completion occurred, registers were:
  ER ST SC SN CL CH DH
  -- -- -- -- -- -- --
  40 51 c8 38 00 00 00  Error: UNC at LBA = 0x00000038 = 56

  Commands leading to the command that caused the error were:
  CR FR SC SN CL CH DH DC   Powered_Up_Time  Command/Feature_Name
  -- -- -- -- -- -- -- --  ----------------  --------------------
  60 00 90 00 00 00 40 00      00:03:34.490  READ FPDMA QUEUED
  61 08 88 48 38 c0 40 00      00:03:31.488  WRITE FPDMA QUEUED
  61 08 80 20 4e c1 40 00      00:03:31.488  WRITE FPDMA QUEUED
  61 08 78 c0 61 c0 40 00      00:03:31.488  WRITE FPDMA QUEUED
  61 08 70 20 3c c0 40 00      00:03:31.488  WRITE FPDMA QUEUED

Error 65533 occurred at disk power-on lifetime: 8005 hours (333 days + 13 hours)
  When the command that caused the error occurred, the device was active or idle.

  After command completion occurred, registers were:
  ER ST SC SN CL CH DH
  -- -- -- -- -- -- --
  40 51 08 38 00 00 00  Error: UNC at LBA = 0x00000038 = 56

  Commands leading to the command that caused the error were:
  CR FR SC SN CL CH DH DC   Powered_Up_Time  Command/Feature_Name
  -- -- -- -- -- -- -- --  ----------------  --------------------
  60 08 80 38 00 00 40 00      00:03:24.703  READ FPDMA QUEUED
  60 08 78 30 00 00 40 00      00:03:24.703  READ FPDMA QUEUED
  60 08 70 28 00 00 40 00      00:03:24.703  READ FPDMA QUEUED
  60 08 68 20 00 00 40 00      00:03:24.703  READ FPDMA QUEUED
  60 08 60 18 00 00 40 00      00:03:24.703  READ FPDMA QUEUED

Error 65532 occurred at disk power-on lifetime: 8005 hours (333 days + 13 hours)
  When the command that caused the error occurred, the device was active or idle.

  After command completion occurred, registers were:
  ER ST SC SN CL CH DH
  -- -- -- -- -- -- --
  40 51 c8 38 00 00 00  Error: UNC at LBA = 0x00000038 = 56

  Commands leading to the command that caused the error were:
  CR FR SC SN CL CH DH DC   Powered_Up_Time  Command/Feature_Name
  -- -- -- -- -- -- -- --  ----------------  --------------------
  60 00 e0 00 00 00 40 00      00:03:24.547  READ FPDMA QUEUED
  61 08 d8 70 52 c0 40 00      00:03:21.545  WRITE FPDMA QUEUED
  61 08 d0 10 38 c0 40 00      00:03:21.545  WRITE FPDMA QUEUED
  61 08 c8 38 a2 84 40 00      00:03:21.545  WRITE FPDMA QUEUED
  61 08 c0 f0 48 81 40 00      00:03:21.545  WRITE FPDMA QUEUED

Error 65531 occurred at disk power-on lifetime: 8004 hours (333 days + 12 hours)
  When the command that caused the error occurred, the device was active or idle.

  After command completion occurred, registers were:
  ER ST SC SN CL CH DH
  -- -- -- -- -- -- --
  40 51 08 38 00 00 00  Error: UNC at LBA = 0x00000038 = 56

  Commands leading to the command that caused the error were:
  CR FR SC SN CL CH DH DC   Powered_Up_Time  Command/Feature_Name
  -- -- -- -- -- -- -- --  ----------------  --------------------
  60 08 58 38 00 00 40 00      01:46:52.737  READ FPDMA QUEUED
  60 08 50 30 00 00 40 00      01:46:52.737  READ FPDMA QUEUED
  60 08 48 a0 23 b4 40 00      01:46:52.721  READ FPDMA QUEUED
  60 08 00 28 00 00 40 00      01:46:52.720  READ FPDMA QUEUED
  ef 10 02 00 00 00 a0 00      01:46:52.720  SET FEATURES [Enable SATA feature]

SMART Self-test log structure revision number 1
Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error
# 1  Short offline       Completed: read failure       50%      8005         56

SMART Selective self-test log data structure revision number 1
 SPAN  MIN_LBA  MAX_LBA  CURRENT_TEST_STATUS
    1        0        0  Not_testing
    2        0        0  Not_testing
    3        0        0  Not_testing
    4        0        0  Not_testing
    5        0        0  Not_testing
Selective self-test flags (0x0):
  After scanning selected spans, do NOT read-scan remainder of disk.
If Selective self-test is pending on power-up, resume after 0 minute delay.

PS: it is not an SSD and is size is 1 To and I have 35.9 GiB free space in my Linux Partition