I think this is the best answer because it emphasizes that this is a real but relatively recent phenomenon. Conspiracy theories were a lot more left/right symmetric as recently as a decade or two ago.

There's literally no evidence towards your speculation and the evidence we do have cuts against it (though I agree it doesn't conclusively refute it). The simplest explanation is that incumbents typically have an advantage while appointees don't. An alternate explanation is that Warnock's advertising was more effective, which is suggested by the fact that Ossoff did better than Warnock in the Chattanooga media market (and other non-GA media markets).

Most of this answer is great, but the speculation about Black Republicans is almost certainly wrong since ballot splitting was more common in whiter precincts

Another big reason the counting is faster in the UK is that they're only voting on one thing. Their whole ballot looks like this! Whereas in the US you are usually voting for on literally dozens of things. That complicates the counting process.