I'm sure you've come across this before and just haven't figured out why it happens. At one point you're fine and dandy moving your camera at regular speeds and then all of a sudden it slows to an annoyingly slow pace.

I think it happens when moving your camera through an object or table.
I have a massive checkers board wall that I can put my camera through and if I zoom in while inside and pull out with WASD, the velocity of a very zoomed in camera remains and you can't zoom in to the table past that Y point anymore.

It's only minor as you can fix this by resetting your camera with spacebar. It's an annoyance though and this bug has been around for a while afaik.