Yes you're right - the first part of the game is much quicker, and definitely the most enjoyable. Once you get to 1000AD, then I usually find I'm engaged in or gearing up for wars with lots of units, which is fun and challenging but really takes ages - as much as 10 minutes just to do one turn. Once I'm at 1500AD, that's when the game starts to get tedious, and it begins to feel like an endless round of, optimising which tiles the workers should improve next, and continuously deciding what to build next in cities that turn out a new building every 2-3 turns. I guess, even though the first turns are fun, at some point I got demotivated from starting games that I knew there was no possibility of finishing. But you're right - maybe I should play a few sometimes just to experience the BC years and then retire
(Of course, if I did that for this game, then I'd be the one risking a conflict of interest if I also have to publish the results haha)