networking - how to troubleshoot asymmetric speed PPTP VPN speed? (downlink to client is slow, the server bandwidth up, and there is no encryption overhead)

I hope I have a general question motivated by my specific problem: I have acceptable upload speeds and unacceptable download speeds on my VPN while I think both endpoints have quite some bandwidth in both directions.

I have a PPTP VPN between a DD-wrt router as server (recent build) and Mac OS 10.8.2 as client, without encryption.

Though the server is on a residential ISP (promising 30 Mb/s down, 10 Mb/s up) a continent away, I still have a link from client to server of 500-1500 Kb/s, and even worse, tragic, appalling 30-70 Kb/s link from server to client.

Bottlenecks on my side cannot be the problem.

Though the router is underpowered, I don't expect that much overhead without encryption.

What is there to hope or do, before I simply blame the DD-WRT build again?