There are some performance differences between X11 and Wayland. Desktop systems are mostly unaffected by them, but laptops suffer a bit.
On Wayland, kwin always does double buffering. This is great for reducing latency and avoiding tearing, but if the gpu can't keep up with the work, you're going to see stuttering.
Another issue is that in order to reduce latency, we need to have very good frame stats. At the moment, kwin records only cpu render time, but we also need to record the gpu time. We've already done some work in this area, but it's most likely Plasma 6 material. (!1163)
In meanwhile, let's change the default latency policy to "prefer smoother animations." It's going to improve frame rate. If people care about latency, they can change latency policy in system settings, the option is still there.