Comment # 14 on bug 1186692 from Uwe Bonnes

More weird:

Background activity may speed up things.

My bisect failed, as bisect step immedate after booted showed slow speed with
several runs. When or after compiling the kernel speed sometime doubled for
several runs. With 
~> uname -a
Linux hertz 5.13.0-rc6-lp152.78-preempt_custom #4 SMP PREEMPT Thu Jun 17
13:46:19 CEST 2021 x86_64 x86_64 x86_64 GNU/Linux

this effect is quite reproducable. Without other activity, read speed is about
50 kB/s. With " yes > /dev/null" running in another terminal, read speed
doubles to about 100 kB/s. Killing "yes ..." gets me back to 50 kB/s

Hints welcome