Quite seriously, the "average user" has higher odds of being struck by lightning than being affected by Spectre!
What is the source of this screenshot ?
In general, Firefox does run *SLOWER* when you enable mitigations that strive to prevent lightning from striking your big toe.
I'm not saying the difference is night and day. But that also depends on the computer you are running.
The older the computer, the more adverse of a slowdown just to prevent something that literally NEVER HAPPENS "to the common user".
Yep, the differences are not clear and not that spectacular. Plus I'm not sure how does it affects our normal, regular daily browsing experience. One more second for loading a page ? Bah!
That said, I wouldn't recommend to anyone to disable these options since we don't really know what could be the side effects. It's up to each user to understand then to take and to assume the possible risks related to these options.