How can I find the speed of a motherboards SATA connectors?

looking in the manual is the obvious way.

sata 0 and sata 1 are the fast ones . dark blue and light blue.

page 4

HP Compaq 8300 Elite, CMT Chassis 690357-001 Illustrated Parts & Service Map

For enthusiasts:

OEM propietary mobos:

It is possible to add the nvme module to bios and boot nvme disk off the pcie x4 slot, but it needs a bios programmer and a SOIC16 SOP16 Test Clip adapter for the bios chip which will be about £25 for both . If you already have those, it is worth it.

Those oem machines have to be done individually.

Consumer mobos:

If you have a consumer motherboard from gigabyte, asus , msi etc, and you want to add nvme boot, the bios with nvme support can be passed around. If you can't be bothered doing it yourself, plenty of people are sharing them.

It is then very straightforward. Plug a cheap pcie>nvme adapter into the pcie x4 slot.