BIOS – Enabling the 16 Lane PCIe Slot on Lenovo ThinkCentre Edge 71

You should put it in the m.2 nvme Slot. The 960 Evo uses at maximum 4 PCIe lanes, and it is debatable whether a GPU actually performs significantly better using 8 or 16 Lanes. This means that slowing your GPU by using the PCIe m.2 is unlikely. I would advise against a PCIe card for your 960 Evo if your motherboard already has a specific slot for your NVMe drive.

Depending on the number of lanes that your CPU (what model?) supports, I would even still use the M.2 slot.

The only issue with the SATA is that if you put a M.2 SATA drive in the m.2 SATA (not the PCIe x4) slot, you will lose access to two SATA ports, port 0 and port 1, as specified in your manual.

Your motherboard has 6 SATA ports, so you would only have access to four, if you are using more than four extra drives as well as the m.2 drive, then this is where it would become a problem.

The 960 Evo isnt a SATA drive, it is an NVMe device, so the SATA information above is just for reference

The Gigabyte B550M DS3H (rev. 1.x) has the information that was asked:

The PCIe 4.0 x16 port can theoretically deliver up to 31.5 GB/s, while the PCIe 3.0 x16 can do up to 15.8 GB/s.

You will find below the theoretical limit of all ports together (again, very theoretically):

While Gigabyte boasts that theoretically this design delivers up to 64 Gb/s data transfer speed, it should be remembered that when launched, this was the cheapest and the lowest-level motherboard in the B550 product stack.

It's also normal for the highest-performance ports to have lane priority over the others, so devices connected to lower-performance ports, when the higher-performance ports are active, may perform at a fraction of their speed (this can be as low as one-half and one-fourth - example).