Vagan (and Haku before the patch) were both a total bitch in merciless early on.
Once your character reaches endgame and obtains high dps though, his quests are mostly quick and easy.
The ones that annoy me now are when Zana tells me to go into a special map and do some objective in 90 seconds. Then when I zone in, the mods are foolishness like no regen + reflect or whatever. I can handle those map mods, but they require me to swap around gems and/or gear first, which wastes the timer. On top of that, there's usually one or two mobs left up hidden in the zone when the time is running out. Screw timed master quests in general.