One of the pre-release versions of MUSHclient 5.06 introduced a bug where drawing of lines in the output window became about 30 times as slow.
In particular commit 9d4f265.
This is how to get pre-release versions.
This was eventually noticed by Fiendish (I’m not sure how he spotted it). I suggest that if you have an earlier pre-release version of 5.06 you upgrade to the latest one. If you have an officially released version of 5.06, or a later version, then this will not apply to you.