I use one as a router, connected to a UniFi AP AC LITE access point (via a PoE switch). It's a very reliable configuration and works fine.
The VMG1312-B10A is relatively old hardware and probably not as powerful as newer routers. I never experience any slowdown, but I don't do anything which puts the router under high load (mostly just "normal" internet traffic and video streaming). Not sure how it would cope with something like BitTorrent which uses a lot of simultaneous connections.