I too updated from 2017.1 to 2017.3, and found out that the editor is lagging very very badly. On regular intervals the editor completely freezes becoming unresponsive. This problem didn't existed in 2017.1. Take a look on the profiler: See all these bumps / mountain tops on CPU Usage ? At each of them, the editor completely freezes for a moment. But that happens in general, not only in play mode. The editor becomes completely unresponsive, every second or so. Plus I noticed that the Garbage Collector is very active. It might be it. Anyway, I rolled back to 2017.1 for now.