I'm pretty sure that up to about a year ago I could enable CUDA within Blender on my MacBook Pro within the Preferences. Recently though the only option I have for rendering in cycles is either a 'None' or 'OpenGL'. My NVIDIA GeForce 750M Graphics card is available with a check box, when the latter is selected. However according to the manual Open GL needs an AMD processor. Mine is an Intel Core i7. GPU rendering does not seem any quicker than CPU. Can anyone else on a Mac Book Pro get CUDA to work in Blender? I'm on High Sierra 10.3.3.
Did you change the tile size when rendering with the GPU? GPUs like a big block of data to render versus smaller chunks for the CPU.
I got some marginal improvement by going from 64 x 64 to 480 x 270, (so thanks for that). I tried several other options and this seemed the best for the GPU. However at no time was the GPU ever more than 30% quicker than the CPU. I just don't think the GPU or CUDA is being used to full advantage, (or the CUDA at all). Maybe I'm just expecting too much from a laptop.
Did you install a CUDA driver? You need to install this to enable CUDA being run on your
GPU. It is not always installed by default.
You can find the latest version (for Mac) here http://www.nvidia.com/object/mac-driver-archive.html.
That's a good start, but I don't think that's a power of 2. Computers do their best computing in powers of 2, so like 2^x, or 2, 4, 8, 16, 32, etc. I've found 256x256 works well for me, maybe 512x512 could work, too. Other than that, I don't know what to tell you. Hopefully someone will come along and help. Just keep trying your best and hopefully an answer will appear. Best of luck!
Your problem is that default MacOS High Sierra Graphics Driver does not support CUDA
You can follow this topic to resolve it
https://blenderartists.org/t/tell-you-how-to-setup-cuda-for-macos-high-sierra/1123391