The devices attached to the 2.5G switch, will be limited to 2.5G per connected port. There won't be any slow down on any other connections. If you still want 10G in the basement, put in a 10G SFP+ switch (they can be had for $15-20 per port and use DACs to control the heat some). You'll only have one shared 10Gbps to the rest of the house, but that is really unlikely to be saturated.