What is happening with high load average but mostly "wa"-wait?

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Among several other annoyances since upgrading from F24 to F25, it is way too easy to get the system to stop responding.

The disk light is on solidly with only a wink (10 ms?) off every few seconds. I can't log in remotely (times out trying to connect) nor switch to an alternate console for diagnosis, but this time (after dozens of tries over the last 1.5 months) I finally got top to show this. Note the load average is climbing with one process running (presumably top) and 92% to 99% "waiting" (disk i/o wait?) cpu time shown each update for the duration.

top - 21:51:00 up 3 days,  6:28,  1 user,  load average: 6.62, 5.43, 2.84
Tasks: 200 total,   1 running, 198 sleeping,   0 stopped,   1 zombie
%Cpu0  :  0.3 us,  0.0 sy,  0.0 ni,  0.0 id, 99.3 wa,  0.0 hi,  0.3 si,  0.0 st
%Cpu1  :  1.3 us,  0.3 sy,  0.0 ni,  9.0 id, 89.4 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem :  4037464 total,   230700 free,  3256972 used,   549792 buff/cache
KiB Swap:  4063228 total,  3690424 free,   372804 used.   219536 avail Mem 

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND     
 1588 wally     20   0 2235276 287484  32592 S   1.0  7.1 114:30.20 gnome-shell 
  327 root       0 -20       0      0      0 S   0.3  0.0   0:08.46 kworker/0:+ 
  899 boinc     30  10  350816   2328   1656 S   0.3  0.1   2:59.09 boinc_clie+ 
16370 wally     20   0 4234612 2.212g  41828 D   0.3 57.4 261:08.08 firefox     
27838 wally     20   0  155796   4084   3620 R   0.3  0.1   0:00.07 top         
    1 root      20   0  215140   3524   1056 S   0.0  0.1   0:06.12 systemd     
    2 root      20   0       0      0      0 S   0.0  0.0   0:00.13 kthreadd    
    3 root      20   0       0      0      0 S   0.0  0.0   0:01.23 ksoftirqd/0 
    5 root       0 -20       0      0      0 S   0.0  0.0   0:00.00 kworker/0:+ 
    7 root      20   0       0      0      0 S   0.0  0.0   0:45.58 rcu_sched   
    8 root      20   0       0      0      0 S   0.0  0.0   0:00.00 rcu_bh      
    9 root      20   0       0      0      0 S   0.0  0.0   0:15.13 rcuos/0     
   10 root      20   0       0      0      0 S   0.0  0.0   0:00.00 rcuob/0

I got the system to recover by closing Firefox which had been rendering two tabs which appeared blank, one containing this and the other this. After closing and waiting for the disk light to go off—taking 30-40 seconds—top shows

top - 21:58:00 up 3 days,  6:35,  1 user,  load average: 0.49, 2.56, 2.53
Tasks: 191 total,   1 running, 190 ...