Vehicle movements killing framerates

Hi,
I'm making a delivery mission where you move 15 of the giant ball props to a destination by pushing them.
So far the concept works fine.
However, for some reason the farther I get into the mission the more my framerates drop and eventually drop as low as 2 or 3 FPS!
I originally thought it's because of so many dynamic objects or delivery detection area may have been too big (@30m), but removing half of them made no difference nor did reducing the delivery zone size.
Then, by chance I noticed that when I'd stop the vehicle (the offroad sunburst) the framerates would return to normal. Start driving again and framerates drop again.
This isn't making sense to me.
Is this a bug or is there something I can change in the mission that would fix the issue?
BTW - I'm generally not experiencing any drop in framerates other than in this instance.

[EDIT]
I tried some different things to see if they had an affect on this...
I made the "delivery zone" smaller and set it up as a "pass through area" so the balls each trigger the delivery as one, then as they move out of the detection area go back to zero. I have noticed doing this that the framerate drops only occur when the vehicle is moving and a ball is inside the delivery trigger zone. (I know the trigger zone has to be checked every frame to provide fair scoring, but can this be somehow optimized) I assume I can design this mission using flowgraph in a way that won't tax the GPU by deactivating each object as it enters the zone, but I haven't got there yet.
Any devs or creators know a way around this? or is it something that can be optimized in the game code in the future?