Bandwidth limitations depend on the following:
Processor performance must be comparable with graphics card.
Limitations of a dual core processor (Core i5, i7) the NVIDIA GTX 960, GTX 970, and GTX 1070 are maximum bandwidth devices.
There are no limitations for quad core and above processors (Core i7 and Xeon)
All calculations are accurate when measured real-time.
For some applications render calculations do not matter as to which version of processor and port you have. Data transfer to the GPU is calculated and returned back with results. The slight delay is not so important and GPU power and acceleration is the variable being considered most important; therefore with older Mac models you are able to harness the power of GPU acceleration at the same rate newer Mac models are.
Examples of bandwidth limitations:
MacBook 2012, dual core i5 is limited to: GTX 960, GTX 970, and GTX 1070.
Retina MacBook Pro 2013-2015, quad core i7 has no restrictions.
These figures are for reference only and depend on the operating system, Mac configuration and software.