Is there a reason to use a dedicated switch with a home router?
My work gave me a L2 switch because they were going to toss it out. Is there any reason for me to use that over the built-in switch from my ISP's router/modem?
A gigabit switch is going to give you the full 1Gbps between any two ports on the switch. Same with 2.5Gbps and 10Gbps switches. You should see very little variance between different switches.
Usually the only time you'll find slower speeds is if it's a router rather than a switch, and the ports are all routed. This is uncommon, and for example a 5-port router is usually really a 4-port switch plus a router in a single device.