Lag in game can be caused by a fragmented gw.dat file. To fix this, you can run a defragmentation tool. Unfortunately, not all of the defragmentation tools available will properly defrag your files. Thankfully, there are several excellent freeware programs to help you defrag your computer's hard drive.
Before you begin you should run CCleaner to remove unnecessary files and data, such as temp files, browser cache and other data that may not be of any use to you. If you need help you can refer to this article.
Now you can use a defragmentation utility. There are several good choices to use.