Here are some console commands you can try out in the demo (from this link NVIDIA VXGI Integration - Unreal Engine Forums):
I found that setting stacklevels to 1 and then changing range to 600 gave me a good base to go from. Reducing map size to 64 or 32 got me up to over 60fps on a 1080. Playable frame rates with a map size of 64 or 128 is definitely possible.
Changes the size of the voxels. Lower (meaning bigger voxels) is better, but increases leaking and inaccuracies. 256 is unusable in most GPUs today. Although Nvidia recommends 64 for better performance, you’re likely better of setting this to 32.
As it says, changes the range. This has a side effect however. If you increase this range, it also scales the voxel size, it tries to keep voxel count constant so that you don’t get bogged down by too many voxels.
As mentioned by Blakbit in this post, this setting has large impact on performance. VXGI reduces voxel resolution for geometry farther than VXGI Anchor (by default attached to camera). Default setting has 5 levels of different voxel sizes. By reducing stacklevel count, you can increase (or decrease) performance. I recommend reading the linked post for more detailed info. Gist of it is you need to use this setting in-conjunction with mapsize and range settings to find a sweetspot that works for your scene.
Enables multibouncing as the name suggests. According to Nvidia overview, it works by using previous frame’s irradiance map in the current frame. Nvidia overview here (requires github access) says disabling it increases voxelization performance.
Mentioned in Daniel.Wenograd’s post here and the overview above.
Mentioned in Daniel.Wenograd’s post here. This affects the multibounce. As he states few posts below, this is related to VRAM usage, more info here.