Did you hover the buff reward?

The 20 hour mission is for a 400% speed boost.

400% increase in spawn speed doesn't sound too bad. But nevertheless, won't it be less time consuming to do the mission instead of waiting for 2 days or more so that it disappears?

well, it should be. It's not about instakilling, but how fast the Monsters spawn and with this, how fast you are progressing through the levels. Usually, there is a pause between a single monster or a group spawning. With this, on average I need about 30 seconds for a Level. Using the spawning, this should reduce the time needed. To be honest, though, I might have come across the 400% speed boost Mission, but I don't recall whether it had the hugest Impact and was really 8 times quicker than the average spawn buff.