I wanted to check if this could be an Ubuntu kernel bug, so I checked Debian on:
Linux debian 5.7.0-2-amd64 #1 SMP Debian 5.7.10-1 (2020-07-26) x86_64 GNU/Linux It is slightly faster there, but the results look just as bad: fatrace.log.0oLjK : .FFFFFFFFF - 9 matches missing fatrace.log.0qKTf : ....F..FFF - 4 matches missing fatrace.log.18xyI : .......F.. - 1 matches missing fatrace.log.2Jk3p : ......F.FF - 3 matches missing fatrace.log.38fQg : .......... - 0 matches missing fatrace.log.3k2ln : .FFF.FFFFF - 8 matches missing -- Christian Ehrhardt Staff Engineer, Ubuntu Server Canonical Ltd