We would need a bit more information to make something of this.
What are you doing when this happens, for example?

Marking as dup of 131461, which we can use as a general “g-t is slow bug” 

*** This bug has been marked as a duplicate of 131461 ***

This is not a duplicate of 131461.
This is about gnome-terminal being _sometimes_ slow.

I'm doing nothing particular when it happens. I just switch back to
some terminal and realize it's slow as hell. Sometimes only that
terminal is slow, sometimes all the opened terminals are slow.
And it usually fixes itself after a few minutes/hours.

Well, it is really very difficult to know what's going on here from
your description of the problem...

Are you sure it is not simply that gnome-terminal is being swapped off?
Does the slowness last for a while or is it just the 1st few seconds
or do you get that for hours, as you say?
Does the slowness go away when you turn off transparency?
Do you have accessibility turned on?

Yes I'm sure that it's not gnome-terminal being swapped off. The
slowness indeed lasts for a while. Accessibility isn't turned on. I'll
test disabling transparency the next time the problem happens.

Oh and I guess it's probably a vte bug rather than a gnome-terminal bug.

Reporter: Are you still seeing this? Can you provide a way to reproduce this? 
As things stand, I don't see anything thatcan be done about this :/

I think it did not happen for a while now (perhaps since I've upgraded to vte
0.11.11). I'm closing this one.