My favorite feature so far is the fact I can switch the editor to release mode, which produces optimized assembly code, but I can still run the profiler on it. That means I get the same timings I would get with a full build, which takes my PC about 6 minutes to make, so I don't. I've been spending a lot of time optimizing and cleanup up lately, so a very nice feature.
I just finished this one, which is a big win:
Optimized terrain control textures to not update with every mesh update.
Those peaked at 2ms spikes on my PC, on several frames, when new chunks load, adding to