[Farmer]

I think what slows down combat the most, is players (and sometimes GMs) not being ready when it's their turn. While the other characters are having their go, players should be working out what they want to do and checking their own modifiers if they don't already know them. Having a little note of your common ones helps, or using a VTT where you can build in special attacks or modifiers etc. also helps.

If everyone has to wait for each player to start working things out when it's their turn, it slows down. Of course, combat is dynamic and things change, but most of the time you can save time by prepping before it's your turn.

And looking at what might be considered simpler systems, like having no defence roles in D&D, I always feel hamstrung and a little powerless when being attacked (unless I'm a wizard or something who can cast defensively).

Some good ideas above about how the GM can run the NPCs, particularly if there are a lot of them - it also avoids mooks critting against the PCs which is often not ideal for the narrative.