Don't know how much this will work but make sure your Java is up to date. Again I doubt thisis the issue but it's good to have your java updated anyways
Another thing is, make sure your anti-virus programs aren't running a scan at the same time, that might be the cause. In which case it will go away once the scan is complete and you have dealt with the results. (Let's hope you have antivirus)
Another program being up and doing stuff at the same time can lag a computer.
Try out that stuff and than we'll see. I doubt it'll work but it's worth a shot. I personally prefer Alice 2.0. It doesn't lag for me and here's my specs:
Windows XP Professional Version 2002 Service Pack 3
HP 5750, HP 5850 (don't know what this is. Came up on the My Computer->Properties Menu)
AMD Semperon(TM) Processor
LE-1250 (Again. Just on there not for sure)
2.19GHz, 768 MB of RAM (Random Access Memeory)
Physical Adress Extension (No clue)