It might be that your card is old or sometimes if you are running other programs in the background then you may experiene a bit of lag.
I've heard you can get lag if you don't have enough ram (even 8 gb). Also how many beatmaps do you have saved and how bad is the lag when you pull up the chat window. So far I have never seen a fix for this. Also try setting FPS to unlimited.
I did find this however,
"There are several reasons why this may happen, First thing that comes to mind is GPU power states kicking down depending on usage.
This is may be controlled by the driver, hardware onboard, and or programming of the game or both like for example a frame limiter on the user panel which causes the gpu power states to kick due to lack of usage (which I do believe is actually implemented for chat & userpanel when shown coulda sworn I saw it months ago in the changelog)."
here is the original thread:
p/2330625