External GPUs on Farm

I am looking at upgrading my render farm for use with Blender.

I currently have 6 PCs all with out GPUs so I am finding renderingwith Cycles on CPU pretty slow.

4 of the PCs are not able to take a GPU, they are small formdesktops. I was wondering if anyone has any experience with externalGPU cases? I was thinking for the 2 full size PCs I could installGPUs but for the others I would need to either get External GPUs forthem or just look at buying more PCs that can take GPUs.

Do external GPUs run as fast as internal GPUs?

Has anyone done something like this before and can let me knowthere experience.