It's probably packet loss, or one end's upload speed is very slow.

ADSL and cable (or any asymmetrical) connections are not well suited for VPNs.

Trying pinging (more than the 4 done by default by Window ping command) the IP address of each side from the alternate and see if there is any loss, or large deviations. Running the ping while the VPN is up may also be very telling.
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