Edit MattK: I think the simplest and safest solution to this is to simply prevent all input from leaving a running play in editor viewport.
A user wants to bind CTRL key to crouch, and when they use the CTRL+S combination to crouch move backwards, it conflicts with the editor's "save all levels" binding and creates lag and causes a crash.
This only occurs so far in "Play in New Window". It's not necessary to bind input commands to repro the issue, but this is a legitimate user case.
1. Run ue4editor.exe
2. Choose Play in New Window
3. enter the stat unitgraph command
4. Press CTR+S while in the game
RESULT: There are FPS spikes
WORKAROUND: This doesn't occur if you unbind CTRL+S from the "save all levels" command. Also it doesn't occur in Play in Viewport
Head over to the existing Questions & Answers thread and let us know what's up.