My NAS has 2x 2.5G NICs and my PC has one, and other than that everything in my network is Gigabit.
What I did is bridge both ports on the NAS, connect one port to the network and the othed to the PC. This way I have 2.5G between my PC and NAS, but the PC still has internet connection thanks to the bridge, all without the need for a 2.5G switch.
My NAS has 2x 2.5G NICs and my PC has one, and other than that everything in my network is Gigabit.
What I did is bridge both ports on the NAS, connect one port to the network and the othed to the PC. This way I have 2.5G between my PC and NAS, but the PC still has internet connection thanks to the bridge, all without the need for a 2.5G switch.