The glib answer to 'how did you solve similar issues' is 'our high-level 3e campaign ground to a miserable halt due to system-breaking maths and option overload and we didn't start playing again until 5e and covid lockdowns came along'
More seriously, one thing that's occasionally worked for me in this sort of situation is to send out a group email asking everyone for ideas and feedback on how the game could be improved or made more fun. This makes it less likely that the slower players will think that it's all just you picking on them, and they get the chance to discuss how the game is working for them too. If your faster players (and you) talk about combat speed being an issue, then you can ask the entire group for ideas about how to speed it up. And this includes your slower players, because maybe they're finding it frustrating as well.
Basically, make it a collaborative effort to improve the game for everyone, not just the DM imposing rules that certain players might feel disproportionately targeted by.