I think the most helpful thing is to look at your scene in the Lumen overview mode.
It should look like the actual scene, roughly.
If it doesn’t, then you have a place to start ( the meshes will be highlighted ).
Lumen has had a lot of tweaking, which by default, will make things a lot slower.
I had a problem with the new surface cache. Lumen had decided to include the leaves on some hedges in its calculations. Once I unset ‘can affect distance field lighting’ the problem pretty much went away.
I did not want the leaves incorporated in the calculation 