I also find that the search is often slow. I don't know if it has to do with how busy the servers are at the time or the phase of the moon. For planning "random" encounters I use a couple of the encounter generators available on non-Roll20 (free) sites to come up with the monsters or NPCs, then go back to my game in Roll20 and search for those specific monsters or NPCs in the compendium. I find that although doing it this way requires more steps and some off-site resources, but it's still faster than trying to search through the complete list or trying to use the built in filters.