A friend recently setup openvpn to run in his router (I'm going to guess it was MIPS based), and his download bandwidth went from ~100Mbps to about 10Mbps; so a

A friend recently setup openvpn to run in his router (I'm going to guess it was MIPS based), and his download bandwidth went from ~100Mbps to about 10Mbps; so a 10x drop. Why? Is a MIPS CPU in a consumer off the shelf router ($200) itself just too underpowered for this task?