Nothing is that simple.
Indeed. However, it is fair to say that no manned asset is going to be doing CAS in a sophisticated air-defense environment (even a Pantsyr-type threat) until the defenses have been taken down, and that (so far) fixed-wing platforms with DIRCM have been able to defeat MANPADS at ranges that are compatible with their sensors and weapons. This of course is irrespective of RCS.
A-10 max speed: 381 kt
F-35 max speed: 1,058 kt
Is there a prize?
Yes. A conical hat and a stool in the corner. If the base is not close to the target (in airpower terms - under 200 nm), an F-35 will scream in at Mach 1.6 and say "sorry chaps, need to refuel, good luck, bye". As will pretty much any FJ at max speed. I think you know that.
Conversely, where the A-10 outpoints FJs is persistence over the target, or in the area. I don't see the 30-mm. as being vital in the era of PGMs or in the coming era of guided tube rounds; but the Hog will have its uses in a small-arms-and-MANPADS environment until another CAS platform comes along. By the way, small gunships and light attackers have their uses too - turboprops and DIRCM make a tough target for MANPADS.
But what you do need for CAS, in addition to a diverse PGM magazine, is multi-band EO/IR targeting and a Rover-equivalent video link. So let's continue this discussion when the F-35 has these things, which on current plans may start to happen in 2023.