The access to the SAP Portal for a certain category of users is very slow.
Does any one has any idea of how to set the bandwidth on the portal and by script in order to improve access performance for low rate transfer users ?
which group of users are you referring to? How are they accessing the Portal?
The bandwidth cannot be set in the Portal, that's an infrastructrue issues. Thus, settings would be required for your load balancer, etc.
Which SPS level are you on? Starting from SPS14 you can build your External Facing Portal, which might be an option for you. Traffic is reduced with this version since only required Portal services are loaded (and some other performance enhancements). You can check out some info on it on http://service.sap.com/nw-efp