what is nvme speed difference of fitting it to an m.2 slot or a PCI-e slot with an adaptor?

NVMe is PCI-Express so theoretically they should be the same speed as long as the M.2 and PCI-E slots are the same generation and your PCI-E lanes aren't already saturated by other devices.

if you are going for speed, avoid adaptors, which can add an unnecessary step, and NEVER increase speeds. adaptors are for purposes of accomodating slots, nothing more.

PCI-E itself is faster for native SSDs than m.2 is (slightly), just like an NVME m.2 is faster than a regular one, and both are faster than SATA SSDs. the pricing usually reflects the speed difference. supply and demand, you know.

None. M.2 is just a specialized PCI Express slot. It's actually possible to use an M.2 slot for any PCI Express device. You're just changing the form factor.