If you use OpenVPN then depending on the cipher used then yes it will cause your "observed" data bandwidth/throughput to drop significantly as the router struggles with the on-the-fly encryption.
PPTP doesn't really use a lot of encryption and so shouldn't really put a lot of strain on the router. Unless you need a VPN on the entire internet connection though, I wouldn't recommend using it. PPTP is also vulnerable and so you should migrate to something like OpenVPN.
If you use OpenVPN then depending on the cipher used then yes it will cause your "observed" data bandwidth/throughput to drop significantly as the router struggles with the on-the-fly encryption.