The best way to fix your signal is by updating your PRL. This will allow you to utilize all of the networks at our disposal - Alltel, Sprint and Verizon. Without any roaming charges.
http://www.the-answerman.com/sidlist/prl/index.asp
Upgrade to 40051 and you should have better signal.
Regarding the lagging, check and see what is actively running on your phone. Close any extra programs that are running in the background. Run Oxios Hibernate (or any other programs that free up RAM). If that doesn't work, you may need to uninstall apps that you don't use. Lastly, you can either hard-reset or flash a new ROM with better memory management.