|If you have an N router then understand that the wireless adapter that is G will shut the whole router down to the G speed...it will only become as fast as the lowest protocol using it!|
Try updating the drivers to the adapter that you believe is causing the issue. Sometimes going back to an older driver will enhance performance.
Sometimes the adapter itself just wears out and like humans slows down :) try seeing is an usb wireless adapter installed to the same computer still has the same issues...if not then you have found the problem. If it is still an issue then there is some application that is blocking the connection and causing the problems....make sur it is clean of all spyware/malware/viruses and programs running in the background using all the bandwidth such as maybe a seeding connection from a torrent download that keeps trying to start.
Use winpatrol (free) to stop all start-up programs and keep them from accessing the adapter till you say it is ok.
Change Is Good