Mojave vs. Catalina was a night and day difference. Granted my machine is one of the oldest allowed so my difference may be greater than others, but mojave had bugs that just slowed things down. Catalina made things feel more lightweight (kind of like going from leopard to snow leopard was).It seems the same to me as far as speed.
I did have some hiccups on the first upgrade but it only involved my desktop screen popping up again after it had already shut down.
It did end up doing a final shutdown with no problems.
The symptom did go away after a couple of the updates.
All is well now.
Also, for those worried about thunderbolt only, what USB devices are you using on the regular with a laptop? If anyone is hanging storage off these, look into a network attached storage device (NAS) and never worry about externals again.