I encountered this bug months ago but I wasn't sure if it was just UT4 or Mesa.
Before I would see 3 to 5 second freezes when a new effect was being rendered
for the first time. A new effect being a weapon firing, an impact occuring
(explosions etc), or even the alternate fire on the enforcer would cause it as
well even if I've use the normal fire previously. Needless to say, this made
the first 2-3 minutes of any match unplayable as an enemy player would enter a
room using a weapon I hadn't 'loaded' previously and I would be dead without
being able to react or even fire a shot. It was a Mesa update a couple months
ago that reduced the freeze time to about 1 second. It's still pretty
horrible.