Why does the hdd read speed drop some time after connecting it?

Question

Hard disks

I'm trying to recover data from a broken hdd.
I use the ddrescue utility (in fact, this is not important).
The strange thing is that immediately after connecting the disk, the speed of reading from it is several Mb / s. but after 20-30 seconds, it drops to a few bytes and does not come back. After turning off and reconnecting the disk, the speed again increases by 20-30 seconds.
Here is smartctl

sudo smartctl --all /dev/sdd
smartctl 6.4 2015-06-04 r4109 [x86_64-linux-4.3.3-2-ARCH] (local build)
Copyright (C) 2002-15, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF INFORMATION SECTION ===
Model Family:     Western Digital Blue Mobile
Device Model:     WDC WD2500LPCX-24C6HT0
Serial Number:    WD-WXB1A549X3UT
LU WWN Device Id: 5 0014ee 65a155052
Firmware Version: 02.01A02
User Capacity:    250,059,350,016 bytes [250 GB]
Sector Sizes:     512 bytes logical, 4096 bytes physical
Rotation Rate:    5400 rpm
Device is:        In smartctl database [for details use: -P show]
ATA Version is:   ACS-2 (minor revision not indicated)
SATA Version is:  SATA 3.0, 6.0 Gb/s (current: 1.5 Gb/s)
Local Time is:    Tue Jan 19 12:09:54 2016 EET
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:      (   0)	The previous self-test routine completed
          without error or no self-test has ever 
          been run.
Total time to complete Offline 
data collection: 		( 4560) seconds.
Offline data collection
capabilities: 			 (0x7b) SMART execute Offline immediate.
          Auto Offline data collection on/off support.
          Suspend Offline collection upon new
          command.
          Offline surface scan supported.
          Self-test supported.
          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: 	 (  56) minutes.
Conveyance self-test routine
recommended polling time: 	 (   5) minutes.
SCT capabilities: 	       (0x7035)	SCT Status 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     0x002f   100   253   051    Pre-fail  Always       -       0
  3 Spin_Up_Time            0x0027   100   253   021    Pre-fail  Always       -       0
  4 Start_Stop_Count        0x0032   100   100   000    Old_age   Always       -       2
  5 Reallocated_Sector_Ct   0x0033   192   192   140    Pre-fail  Always       -       369
  7 Seek_Error_Rate         0x002f   100   253   051    Pre-fail  Always       -       0
  9 Power_On_Hours          0x0032   100   100   000    Old_age   Always       -       0
 10 Spin_Retry_Count        0x0032   100   253   000    Old_age   Always       -       0
 11 Calibration_Retry_Count 0x0032   100   253   000    Old_age   Always       -       0
 12 Power_Cycle_Count       0x0032   100   100   000    Old_age   Always       -       2
192 Power-Off_Retract_Count 0x0032   200   200   000    Old_age   Always       -       1
193 Load_Cycle_Count        0x0032   200   200   000    Old_age   Always       -       1
194 Temperature_Celsius     0x0022   114   113   000    Old_age   Always       -       29
196 Reallocated_Event_Count 0x0032   001   001   000    Old_age   Always       -       369
197 Current_Pending_Sector  0x0032   198   198   000    Old_age   Always       -       95
198 Offline_Uncorrectable   0x0030   100   253   000    Old_age   Offline      -       0
199 UDMA_CRC_Error_Count    0x0032   200   253   000    Old_age   Always       -       0
200 Multi_Zone_Error_Rate   0x0008   100   253   000    Old_age   Offline      -       0
240 Head_Flying_Hours       0x0032   100   100   000    Old_age   Always       -       0

SMART Error Log Version: 1
No Errors Logged

SMART Self-test log structure revision number 1
No self-tests have been logged.  [To run self-tests, use: smartctl -t]

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.

Yes, I know that it's time for the disk to go to the trash, so I'll emphasize - I need to restore the data , and some of the data has already been restored. The process slows down the decline in reading speed. Maybe linux switches it to some mode after a few seconds? Add cooling? What else can be?
UPD : I will not give it to specialists. I'm learning myself. The information is not important.

Answer 1

The speed drops because the screw starts error handling.
To turn it off, you need a special software and experience.
If you really want to do it yourself, then the cheapest thing is to buy WDMarvel for a month wdmarvel.com/ru But what
's the point for the sake of one screw?