I compared pptp performance between i386 and amd64 versions of ubuntu 9.10. My cpu e8400. Config files was the same, network manager completely removed. I found on amd64 pptp as twice slower as on i386 (avg 5,39M/s on i386 and 2,78M/s on amd64). Near windows pptp server (connected in one giga stacked switch) on ubuntu 9.10 i386 without pppoe i have 8,93M/s over 1gbit link on Intel(R) Pentium(R) 4 CPU 3.00GHz.
My pptp connection established over pppoe, but in both cases pppoe transfer data with maximal speed (100mbit, tested with wget -O /dev/null).
pty "pptp <my_ip> --nolaunchpppd "