VPN Performance

We have a client that wants us to do an offsite backup for their facility. They have a DSL line in their office. Our problem is that no matter what we try we cannot seem to get enough throughput out of the DSL line. I have tried QOS with no success. When I ping them, I have a latency of 70-75 ms. Our cable internet provider’s latency is 15-20 ms. I would appreciate some advice. I want to make sure I have not missed anything before advising our client they need to switch ISP’s in order for us to do an offsite backup. Thanks for your help!