F-Zero X VC, slow and unstable frame rate at 100% speed

[Tasoulios]

I have this problem with F-Zero X ever since early versions 4 of Dolphin.

Emulation speed reports 100%. VPS is steady 60. CPU usage is low-ish.

But the game runs at an unstable frame rate with slowdowns. Mostly 45-50 fps instead of 60 (US version).

So why is the game so slow but Dolphin claims it runs at 100% speed?