Slow UPDATE vBulletin - what's the catch?

I bought a license for vBulletin for two forums. On one - it flies around the clock (about 20,000 registered users), on the other - it begins to slow down in the evening (120,000 users).
Attendance is the same - 50-100 people online. Launched on the debugging forum Debug Mode , it turned out that the latest request takes up to 16 seconds!

UPDATE user
SET lastactivity = 1226505039
WHERE userid = 1

* This source code was highlighted with Source Code Highlighter .

Moreover, once it is not necessary at once - the request can be executed a couple of times normally (thousandths of a second), and maybe 16 seconds.
I think that it would be wrong to blame the size of the database and the number of users, although the idea was to delete those who did not attend the forum.