Framerate drops to 1fps in editor after play a few times

This never used to happen but now it's a certainty. After pressing the play button a few times (sometimes even as little as 2 times) then the framerate gets very bad for VR. It progressively gets worse until the game runs at 1fps. However this is just in the editor and not in the build.

It's so annoying and time consuming. The only way I can fix it is to close the editor and open it again. This is extremely crap because the way I make things is to do a small piece then test, then keep going (so as not to accumulate oversights and bugs and to make general tweaks before going too deep).

I don't know why it just started happening one day but has been like this ever since.

I'm using Unity 2017.3.0f3 (64-bit)

 

Did you use the Unity Profiler to find our what's causing this slow-down already? Here are a few resources that should help you to get started with the Unity Profiler.

Learn how to diagnose common performance problems and optimize your projects:
https://unity3d.com/learn/tutorials/topics/performance-optimization

Spoiler: Introduction to the Profiler - Unity Official Tutorials

Spoiler: Unite Europe 2017 - Practical guide to profiling tools in Unity

Spoiler: Unite Seoul 2018 - When optimization goes wrong

PS: Upgrading to Unity 2017.4 probably wouldn't be a terrible idea too.