E-Cycles 2.90 got a massive update with a log of bug fixes from upstream on the Blender side and a switch to CUDA 11 for the E-Cycles side. I really recommend to test it and provide feedback if needed. If all goes well, I will include the performance improvements in the final 2.90.1 release :)
E-Cycles 2.91 got a new feature update. You can now tweak your light groups directly in the viewport while it continues to render! Previously tweaking a light would reset samples count to 1 and viewport resolution to 64x64. E-Cycles 2.91 offers the possibility to create the perfect light setup really quickly thanks to high sample count, full resolution and real time updates!
You can get an example on an exterior my Youtube channel.
And a small tutorial on Youtube too.