Well it depends in your host as to how much strain this hack will place, and which version of UBB.Classic we're talking about... I believe LK mentioned that some of his ealier versions of the hit-hack for earlier UBB.classic versions are less efficent than the current version (ver 2.0). Best way to find out is to install the hack but make backups of the files you have to edit.
If you notice that your UBB is acting slower than before then you can decide whether to un-install the hack. From my experience ver 2.0 seems very stable, works fine and hasn't added any noticable speed loss
.