the IP address is the same as your home IP, or should be if your running your own VPN Server.. so thats moot, its the same IP as if you used your WiFi w/out vpn.. using it over home wifi should not cause any slowdown at all unless your vpn server is very slow at crypto.
if you want to access your cameras when on a cellular connection, it will require VPN connectivity.. leaving the connection open is unlikely to cause any noticeable slow downs at all given the speeds of cellular connectivity.. the only potential downside to leaving VPN running on a cellular connection is if you get some traffic exempt from datacaps, ie netflix, slingtv, or pandora might not count against your data usage; but once you turn the VPN connection on those will start to chew up data.