it actually wouldnt drop performance THAT much if its added in properly. xsi doesnt slow down that quickly and that option is there. it usually takes about 20-30 minutes of constant adding of operations before its slow. so every 30 minutes, freezing a model once, thats not so bad. plus toward the end of the modelling process you are mainly making tweaks to the surface anyway, and thats a minor hit, compared to bevel operations and what not.
i just cant believe that modo doesnt have this in there. well, ive gotten used to it now and dont mind it much, but when it was first released, and being advertized as a “next gen” modeller, without history. almost seemed like an inside joke, where a patch would be released. its definitely gotten a lot better since 101, but i still believe its got too many of lightwaves short commings in there.