If you leave the grass ESPs enabled then they'll be rendered by the Morrowind engine (and I guess MGE as well if you used the ESPs whilst generating distant land). Chances are your frame rate will suffer quite badly.
It's also possible that with grass rendered by Morrowind all those little clumps will have collision as well.
So, really, MGE / MGE XE is needed to use grass in any playable way.
Meh, I don't think the framerate was that bad. You have to remember that the original grass mod was made way before MGE took over the rendering of the grass.
There were issues, the AI sometimes didn't do to well with it in places like the grasslands (big areas of grass and in some cases small creatures) and I seem to recall that load times did suffer a bit. I don't recall having a problem with framerate.