Terrain collider slow framerate at height

Hi all. I have a very large terrain, with a much smaller one that sits above it as an island, on which the player starts. Framerate is normal (30+, my laptop isn't amazing). When the player boards an aircraft and flies up, the framerate drops dramatically to 1 - 2 fps, and on the stats window the Main Thread shoots up from 60 odd to 600 and climbing, the higher you go. If you struggle forwards, it returns back to normal framerate as soon as you're not above the island any more. Also if you return to a lower height, same again. I searched here, and found what I thought was the answer. I lowered the res of the terrain, but there's no appreciable difference. If you disable the terrain collider in-game, the framerate instantly recovers, and comes back if the collider's re-enabled, so it's definately that. None of the other terrain options change anything. As a workaround, I've put a trigger in the air that disables th collider when you're at altitude, but that's not ideal. Can anyone offer any other ideas please? I'll stick a video uo if anyone wants a look. Thanks in advance Ian.