The file 'dl.cgi' is used to generate temporary speed-limited symlinks for each user downloading each file.
For example; you can set your non-premium users to download at 100KB/sec and the file itself 'dl.cgi' will create a symlink (site.com/dl.cgi/abcd/1234.filename.zip) that is capped at that speed.
If you have a lot of users downloading different files  (busy site), then this will naturally cause the CPU to reach almost 90% or 100%
Here are some solutions to help keep the CPU at reasonable levels.
- Upgrade the CPU and the RAM Â (This also helps reduce file I/O bottlenecks)
or
- Upgrade to Sibsoft's Nginx-Mod & Nginx WebServer. This will dramatically free up your server resources during peak periods. Â Â (You can still set the speed-limit for your non-premium users with nginx too.)
or
Otherwise, if you're still using Apache (web-server) and if you don't mind when your non-premium users download at full-speed. Login to your website as admin user and go here: Setting-->User Limits  and  look for the option Generate DIRECT links  and make sure each box is checked  (Free, Registered, Premium)