Maybe with these frameworks threads are less dedicated and instead are more cooperative, idk. Really not my area!
This was back in the Windows XP days when I was working on OpenGL and DirectX. It would do this while rendering like a couple of triangles. One core I could understand, but not all. I'm pretty sure the driver had some spinlocks in there.
I also managed to find out the NVIDIA driver assumed user buffers passed to OpenGL (VBOs) would be 16-byte aligned, using aligned SIMD operations on them directly, even though there's no mention of alignment in the OpenGL spec.
It just so happened that Microsoft's C++ runtime would do 16-byte aligned allocations, while the language I was using only did 4-byte.
All is fair in love and performance wars I suppose...