Severe Lag

Lag is sometimes caused by a network issue rather than an issue on your computer - sometimes. (Make sure that you don't have any torrent clients running or other downloads / uploads / updates etc)

Also, try connecting to the internet from a different internet connection, if possible (hopefully you have wireless). If you do that you might be able to verify whether the problem is with your connection or with your ISP/network.

Is the lag persistent at all points of the game?  Or is it only in the late game?  Constant lag is probably, as Wrath pointed out, due to the network and not your computer...but if you are only lagging late game then it's probably a performance issue...

This may not be relevant at all, but out of curiosity where are you playing?  Like, are you just using your home internet or are you playing off of a buildings WiFi (work, or university for example)?

It seems to happen after a starbase is built.

Well that is rather strange...beyond wild guesses I haven't a clue why that is...is it starbases in general or a certain faction's?

it might be the advents, but I do not know. I used to be able to handle large scale battles, so I didnt think the starbase would be a problem.

This is a complete shot in the dark, but iirc the advent sb got a lot of work in the sins optimization project....could be that sb just doesn't mix well with your computer....could try out the sinsoptimization project and see if that helps