Following up: it looks like this has been resolved in the 4.20.7.0 update.
The change I made was disabling tessellation on the landscape. It was used to displace the ground when snow built up, however having the tessellation enabled at all was having performance implications like this.
Longer-term the plan is to have a snow plane that follows the camera around and provides the snow/mud displacement (without tessellation), but for now the snow will be flat.