Comment # 33 on bug 1112824 from Michiel Janssens

There is a possibility that the spectre mitigations have something to do with
all this, and or hyperthreading/smt.
I tested my system on the impact of those mitigations.
As a reference I used a mouse click on "Show applications" item on the
gnome-shell menu, running wayland. Watching if the application icons transition
smoothly from the corner into the main desktop area.
Hardware: Dell XPS 13 9360
Snapshot 20181118
Kernel 4.19.2-1

My subjective results
1. boot with default kernel parameters
Transition is barely noticeable, not smooth for sure

2. boot with kernel parameters:
pti=off spectre_v2=off l1tf=off nospec_store_bypass_disable no_stf_barrier
Transition is very smooth

3. boot with kernel parameters:
spectre_v2=off
Transition is smooth
This parameter seems to have the biggest impact of all the tested parameters.

Now I read about the new recently disclosed spectre v2 vulnerabilities and the
STIBP mitigation. Some voices say that we might as well disable
hyperthreading/smt instead of these new mitigations.

4. boot with default kernel parameters, and hyperthreading disabled in BIOS
Transitions are smooth.

For the moment I will leave all mitigations enabled and hyperthreading
disabled.
For the record, this is an Intel Kabylake cpu