A lot of scripts perform the command WHO $CHAN upon joining a channel, for the purpose of identifying user addresses and other info. Oftentimes recklessly where it induces a great amount of lag or boots you off the server.
Make sure that you only load your own mIRC scripts that you have personally written or reviewed. Also make sure you're using the latest version of mIRC, which is 7.52 presently.
For scripts that do require this information, it's recommended to use the built-in command /IALFILL $CHAN, spaced 10 seconds apart, and which ought to suppress the output from appearing in your status window.