Same issue here… It looks like it’s just an unoptimized tool (it’s in beta for a long time). You can see CPU usage is very low when calculating, it looks very single threaded.
Disabling lightmaps shouldn’t make any difference. The bottleneck has nothing to do with the mesh itself, but with the algorithms calculating their position based on the procedural rules you created.
One way of improving speed is to reduce the amount of foliage components (less stuff interacting with each other). I’ve created two or three copies of the same spawner volume for different “types” of assets that I don’t care too much if they overlap. Like one for debris and rocks, another for small plants and other for tall plants. But even then I’m waiting for like 1 to 2 hours to spawn foliage on large landscapes.