Poor performance SDL2 - WaitForSingleObject

I'm using SDL2 to create a cross platform, single threaded application, Mostly the development is happening on OSX where in debug i have no performance issues but on Window using much beefier hardware, and more memory (in debug) I have almost unusable performance (around 700-1000 draw calls).

Using the Very Sleepy profiler my biggest drain seems to be from SDL2 interacting with Windows API - WaitForSingleObject (rather than explicitly my application). Have I done something obviously daft here? Any way I can improve the performance?