Have a very similar to someone else on here which also isn't resolved.
My customer's Internet crawls with a DL speed of around 0.6Mbps.
If I bypass the firewall and connect a laptop to the public interface on the DSL router, I get around 7.6Mbps.
This is the same whether there are other client's connected or not so certainly not an infected PC or a user with Bit Torrents.
I have the same ASA config and the same DSL router running on other sites with no problems.
Any diag help very welcome.
The MTU inside and outside is set to 1500, much like the others that work OK.