SimpleRockets 2 has a much better graphics compared to SimplePlanes, so I understand there's definitely going to be some performance degradation. What I don't understand is the impact of part count on performance. My device is an Android mobile, with Snapdragon 855 CPU, 8GB RAM, which is a relatively powerful Android device. In SimplePlanes, with both graphics and physics set to high, I can still reach about 40fps running a 1623 parts craft. However, in SimpleRockets 2, with the overall quality set to medium and refresh rate set to 60Hz, I can only reach about 30fps running a 657 parts craft. Such difference confused me. The performance issue gets ever worse if some parts were separated from the craft when you will experience extreme decrease of frame rate for a few seconds. I don't know what cause this. I hope you guys can deal with these low performance problems and make this brilliant game even better!
If you have part lift forces enabled, that could be the cause. It means fuel tanks and such in SR2 are being computed the same way wings are, where in SP they only calculate drag. It's more realistic, but lift calculations are a lot more expensive than just drag.