amol, it's most probably due to hlsearch support, which scans the whole file for matches in order to highlight them. If you disable it from Tools|Options|ViEmu, it'll probably go away. I also guess it's happening with large files, as it's not very noticeable with smaller ones.

A better scanning strategy, with incremental scanning as you page up or down through the file, should be the *real* solution to this, though. It's a possibility in a future version.

Please let me know whether it's something else (of course, complex regular expressions can cause this too).