Shockingly slow SSD reads on Debian Jessie

I set up a desktop computer after nine months in the attic. I somehow doubt this is related but I don't remember it being this way.

It took six minutes or so to boot, another couple minutes for GUI login screen to come up, and another couple minutes for desktop manager to show.

hdparm gave the following output:

# hdparm -tT /dev/sda
/dev/sda:
 Timing cached reads:     2 MB in  3.75 seconds = 545.64 kB/sec
 Timing buffered disk reads:   2 MB in  4.45 seconds = 459.91 kB/sec

I then ran fstrim like such:

# fstrim -a -v
/home: 23.4 GiB (25130262528 bytes) trimmed
/boot: 397.1 MiB (416373760 bytes) trimmed
/: 5.2 GiB (5628624896 bytes) trimmed

hdparm improved somewhat:

# hdparm -tT /dev/sda
/dev/sda:
 Timing cached reads:   23604 MB in  2.00 seconds = 11811.48 MB/sec
 Timing buffered disk reads:  54 MB in  4.27 seconds =  12.65 MB/sec

smartctl output:

# smartctl -data -A /dev/sda
smartctl 6.4 2014-10-07 r4002 [x86_64-linux-3.16.0-4-amd64] (local build)
Copyright (C) 2002-14, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF READ SMART DATA SECTION ===
SMART Attributes Data Structure revision number: 1
Vendor Specific SMART Attributes with Thresholds:
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  5 Reallocated_Sector_Ct   0x0033   100   100   010    Pre-fail  Always       -       0
  9 Power_On_Hours          0x0032   098   098   000    Old_age   Always       -       7650
 12 Power_Cycle_Count       0x0032   099   099   000    Old_age   Always       -       694
177 Wear_Leveling_Count     0x0013   089   089   000    Pre-fail  Always       -       126
179 Used_Rsvd_Blk_Cnt_Tot   0x0013   100   100   010    Pre-fail  Always       -       0
181 Program_Fail_Cnt_Total  0x0032   100   100   010    Old_age   Always       -       0
182 Erase_Fail_Count_Total  0x0032   100   100   010    Old_age   Always       -       0
183 Runtime_Bad_Block       0x0013   100   100   010    Pre-fail  Always       -       0
187 Uncorrectable_Error_Cnt 0x0032   100   100   000    Old_age   Always       -       0
190 Airflow_Temperature_Cel 0x0032   067   057   000    Old_age   Always       -       33
195 ECC_Error_Rate          0x001a   200   200   000    Old_age   Always       -       0
199 CRC_Error_Count         0x003e   100   100   000    Old_age   Always       -       0
235 POR_Recovery_Count      0x0012   099   099   000    Old_age   Always       -       277
241 Total_LBAs_Written      0x0032   099   099   000    Old_age   Always       -       11876409458

Where do I go from here?

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