[pptp-server] speed issues

[Frank Cusack]

Also, what MTU/MRU are you using?  Try setting it to 1500-16-4=1480.  This
is to account for GRE overhead (16) and MPPE overhead (4 -- which the MPPE
patches you are using (assuming you're doing MPPE) don't account for).

GRE unfortunately has no way to do pmtu discovery so you need to manually
tweak the MTU/MRU to avoid fragmentation.  You can't completely avoid it
since you can't to pmtu, but I think it's reasonable to assume a 1500 mtu.

/fc