Hey CyberGolem,
I believe I can shed some light on this subject a bit more. So we have determined your GPU is the Mac version of the Nvidia Quadro FX 4800 which is a DX10 Compatible card. This also means you are using the SM4 shader model. The default for DX11 uses the SM5 shader model which is the current most up to date model used in the engine.
As you have pointed out for yourself, I believe your issue here is the hardware you are running. Direct3D version 10 is directly equivalent to all OpenGL 3.x versions. Direct3D version 11 is similarly equivalent to all OpenGL 4.x versions. In our Hardware and Software Specs documentation we recommend an OpenGL 4.1 compatible graphics card. This is because compatibility for earlier versions cannot be guaranteed to work as they are a bit older and are becoming less supported. I do not normally suggest upgrading a users machine/hardware unless I believe it to be the issue, and in your case, this has the potential to cause issues across your projects that you create.
With all of that said, I would set your distribution to a constant uniform so there are less variables on Spawn Rate. Then take a look at your Project Settings > Rendering > Frame Rate, and see if you are using any frame rate smoothing or have a min and max frame rate set. This could be essentially limiting the rate of your spawning particle system.
Let me know if you have further questions or need additional assistance.
Thank you,