IME it's truly surprising just how often that +1 or -1 makes all the difference.Small bonuses have almost no effect on a d20 roll, and it turned the game into the dreaded bonus hunt.
I don't mind slowing things down a bit if it adds more variability and-or fun to things; but then again I don't have feats, reactions, etc. which can also slow things down so I've perhaps got more headroom to work with.And that would be another type of bonus hunt. 5e, in particular with unchanging bonuses and adv/dis has divided the length of time needed to run a combat by a factor of 2 to 10 depending on the circumstances, while still keeping exciting, actually making way more exciting because it's so blindingly fast, there is no way I'm going to go back on these principles.
Criticals are quick and fun, and fumbles worse than an automatic miss would just slow down the game again.