In order to test the router simply take it out of the way. Directly hook the computer to the modem without the router in between, if the speeds improve then the router is part of the problem.
Did you run a registry after uninstalling it and reboot? (like ccleaner)
Did you install the latest version? Yahoo! Messenger 9.0.0.2124
http://filehippo.com/download_yahoo_messenger/
It was simply an option when I initially started my computer up. I asked here about it, and decided what the heck, and have had no problems. So figured if it ain't broke, why fix it.