Complex sentence: Pensjö's interest in LPMuds eventually waned in the early 1990s, but by that time LPMud had become one of the most popular forms of MUD.
Split into simpler sentences: Pensjö's interest in LPMuds eventually waned. By the time it did, in the early 1990s, LPMud had become one of the most popular forms of MUD.