1. Check out the messenger's website to see if they're doing something (like maintenance or upgrades) that could be slowing things down.
2. Check the program folder to see if there's a cache file that may be getting too big (causing lag).
3. Wait it out. Sometimes the network may be unusually loaded with traffic.
4. Check your settings. Was anything changed recently?
5. Time for a virus/adware/spyware check. Run'em all, check'em all, get-rid of em'all.
6. Clean your PC. Most lag issues (when spyware, viruses, and network lag aren't happening) are caused by your PC being a junkyard. Defragment, scandisk, disk-cleanup, check your page-file usage, and try archiving some old files you never touch to clear up space.
7. There's a seventh step...but you'll need coffee and at least one magazine before you're ready for that...