This is actually a topic I've been looking for a good answer on for awhile. It's been a problem for me in every civ game, and it's been especially bad with Civ VI. I have a pretty damn good computer - solid state hard drive, plenty of RAM, etc. but no matter what, the game ends up having the time between my turns literally take a minute each once I'm in the medieval period.
I do play on epic speed and have been using 8 Ages of Pace (used to use the mods the slowed culture and science trees only, but find this mod is better as it deals with GP points, wonders, etc. too). I know this slows it some - it definitely means that city states have time to spam units which I believe is part of the problem. However, the map will still have good chunks unsettled by the time turns begin taking forever. Not long after time between turns start taking forever, my actual turns begin to lag as well - a mouse click will be registered a second or two after I make it, the screen will drag...
I've tried going from large to standard maps - though I dislike that. I like having at least 6 or 7 other civs to play against (I prefer 8 or 9 honestly) and a standard map is really too small for that to be fun.
Not sure what to do beyond reducing graphics and resolution, which I have done already.