I haven't played FoV, but there was an RPG called Sacred, in which enemies respawned literally as soon as you left the screen. This got tedious pretty fast, although a lot of that might have been because of the enemies who could Root you to the spot even if you were on horseback and just trying to run through the area.
I don't know the specific circumstances of the respawning enemies in FoV, but if it is reasonable, wouldn't it be better to have them respawn only after you leave whatever dungeon they're in? If it makes sense that enemies are constantly spawning in at that point, then disregard this, although 5 seconds does seem fast.
