vpn - barebones features for a DD-WRT router acting as PPTP server

I have a little router (Linksys E900) only to act as a VPN server. I have a decent router behind it, handling radio anyway. So I am happy to relegate any other CPU-intensive stuff to the bigger router, if it is expected make a difference in VPN performance in the slower one. (I am currently getting much slower throughput, esp. from router to client, than the endpoints' connections could explain.)

What (or where) is a list of features on by default that would be worth turning off?

Keep DHCP, remote management and PPTP server only? What else drains resources?

Thanks!

PS: A relevant DD-WRT forum thread is under http://www.dd-wrt.com/phpBB2/viewtopic.php?p=737253#737253 My SuperUser question about the asymmetric VPN speed itself is under how to troubleshoot asymmetric speed PPTP VPN speed? (downlink to client is slow, the server bandwidth up, and there is no encryption overhead)