Any additional info on this subject? We have the exact same problem...tried disabling the HTTP Compression that was suggested and it didn't fix the issue. Speeds are equal up and down when tested without ISA in the mix...as soon as traffic is routed back through ISA our upload speed is 1/3 that of the download...any additional suggestions would be great.
Just to confirm that it's the web proxy filter, try disabling the web proxy filter on the HTTP protocol, and configure the clients to NOT be web proxy clients. Then disconnect all sessions already established by the test client, and try again. You have to make sure the test client is not a web proxy client, otherwise the connection will go through the web proxy filter on the firewall.
I find that the Web Proxy Filter contained within "Web Filters" within Configuration/Add-ins can be disabled. It is a gateway anti-virus and has been found to hinder certain types of downloads. There is also the Web Proxy Filter found within Application Filters. That cannot be disabled.