I think the lag is common to all JetBrains IDEs: IDEA, WS, PyCharm, etc. IIRC, it's the JVM not using any acceleration for rendering and there might be another bug whereby using the integrated display and an external 4k panel at the same time causes massive lag. Try closing the lid with your 4k monitor attached and see if things speed up.

In any case, I feel you on the system-wide lag. I basically just avoid using JetBrains products at this point.