Since patch 1.66 when you look at a place with a lot of ingame activity, the vehicle gun-zoom does not work as intended.
I tried this in a round of KotH. When I am in base I can zoom in the gunner seat in my cheetah as fast in and out as I can hit + and -.
When I move to a hill where I can look into the city, the zoom is not working as fast and basically it can take seconds until you zoomed in OR out again.
When I tun my turret and look the other way (ocean where nothing is happening) everything works fine.
I tested this in a Cheetah and all friends(4 people) I played with have the same problem.
We have steady fps at the time (mine was 40).