I'm working on a full-length animation, and it's coming out pretty good.
But I noticed that as the .swf progresses the animation lags, and its quite annoying for it makes my animation look choppy.
What can I do to fix this?

if your trying to play the animation in flash, it will start to lag because of the increase of the file size with a longer animation. its right now happening to me with my rhg battle because of how much i animated so far. but if you play with control + enter, it shouldn't lag. if it does, then idk how to help you

File size is most likely the reason
Other reasons could be over use of filters i.e. glow and blur, or yor computer has a bad graphics card

Uploading it too fileize or some other file site should help you