The same for me. I've update to SMP 5.0 after purchase RWC (so 20$ + 20$), and it was better with SMP 4.0.
When I desactive SMP 5.0 I've 50/55 FPS, and It drops to 15/20 FPS, just with broken clouds, without volumetric beta.
I'm very disapointing and hope a future update will improve that.