ANSI color sequences colorize normal text. SMM supports those and if you connect to a MUD supporting it too, you can have colors while mudding. It just looks better.
But as SMM has to scan each line for ANSI-sequences by hand and adjust the colors etc. It might slow down SMM; esp. when recieving large title pages with a lot of ANSI-sequences.
While normal mudding the slow down shouldnt be large and usually computers nowadays are fast enough.
Some may perhaps turn ANSI sequenzes better off while fights (or buy a
faster computer ;) )