Shardstone
Hero
The solos end up having to be semi-set piece or full on set pieces, so they can take between 30 minutes to an hour. At high level, due to the amount of options, the amount of hit points, the saving throws, wild effects, I feel like this is unavoidable.@Shardstone How did those multiple-turn-solo-monster combats run in terms of time spent at the table? More generally how long did your high level combats take?
I've since made other changes to combat to make it more enjoyable. I know you have your own stuff that you write up and run, so just keep in mind that any high level combat needs to be explicitly FUN above anything else, since you'll spend like 45 minutes on average in each one. There's a lot of spectacle and a lot of engaging bits and nobs you can introduce to your encounters to do so, but without these, you end up spending an hour each session bored out of your mind. It's very much a game about "the joys of combat" at high level, way more so then at low level imo.
Naturally there's ways to speed this up; if you embrace the rocket tag nature, you can get combats in under a 30 minutes, and I had some like that. A low survivability but big nuclear attack or effect can be pretty fun from time to time, especially if gets Surprise so that it can try and do its "thing" before getting disintegrated.
I also used minion rules too for swarms. A swarm that changes at certain HP break points can be an encounter by itself with your players narratively taking out dozens of foes as they fight through an army or something. These encounters go a lot faster, usually like 15 minutes max.