FPS Limit causes 100% CPU in one thread - disabling hyper-threading solves the problem

I noticed a weird CPU problem when I was trying to solve stuttering issues.
I 've been using MSI Afterburner and Riva Tuner Statistics Server to display onscreen information about frame time, FPS, etc. All 8 threads of my 6700K CPU are displayed.

19 cars on track e.g. Imola 1972 and 19 Formula Vintage cars - AI driver.
Maximum graphics settings.

PROBLEM
Enable Automobilista's FPS Limit (e.g. 60 or120) - CPU thread 1 often reaches 100%.
Disable Automobilista's FPS Limit, but use Riva Tuner's FPS Limit - CPU thread 1 often reaches 100%.

WORKAROUND
Disable hyper-threading in the BIOS. The CPU usage is now spread more evenly. I can use FPS Limit.
The game seems to run more smoothly, even with all the graphics settings at maximum, but I need to do more testing to check this.

EDIT
I've been using the AI driver so that I can look at the OSD and stuttering. I've just tried it with my wheel. The AI driver must have been pushing up to CPU usage. The CPU does still reach 100% when using the wheel, but less frequently.