Did you try turning on the "Use Half-Precision" ? This makes it possible to be used in hardware that is less than 10GB of VRAM, maybe with this enabled you can achieve grater resolutions and/or speeds because it will have less precision, so less vram is needed. (enabling this will do float16 instead of float32).
If you want more speed, I guess, use the same resolutions with half-precision enabled... I cant test this myself, because I only have 8GB VRAM, and GUI dont let me uncheck it