Web ERP slow response when somebody upload files in an ADSL connected office

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http://serverfault.com – We have developed, and deployed, a web ERP for various small companies (less 30 users each one). Those who have cable connection don't experiment any problems related with slow connections, but one is connected to the internet through 3 adsl lines using a peplink balancer to bond them all; this solution normally works, but when somebody upload a file to the ERP, all the connections from the office that are actually using the same adsl connection to the ERP experiment a lot of slow responses (I know this can happen in adsl connections). The problematic office isn't able to use cable connection (HowTos)