It's a bit hard to measure. Upon loading up the game the effect is observable immediately though. I tried disabling the PLE checkbox and that seemed to speed up things a little bit, but it's still lagging more than without BUG.
Loading it up without BUG and as I said before, the unit selection is near instant and much more pleasing to use. Now I haven't looked into what happens by running the game for longer (an hour or so) without BUG so I can't say whether there is a leak or not.
I am running a huge map and the strain on my computer's resources is probably quite severe at this point in the game. I have 2GB RAM and at least 1GB of that would be available to Civ. Also, the selection lag tends to get worse towards the end of games, as I said before. I'm not sure whether this is because there are more units on the whole map, whether there are more memory requirements of the game or some other reason.
Here is a
savegame I have where the lag is particularly noticeable. Take a look at the island city of Sexi (lol nice name for a city!) where I have a large invasion force almost ready to go. On the city tile there is probably a hundred units. Try selecting individual anti-tank units or ctrl+clicking them to select them all. Try moving them left and right on the 2 tiles of the island. The movement lag also seems to be severe when moving onto a tile occupied by a large number of units.