Full bars doesn't guarantee fast speeds. There are tons of variables that come into play with internet connectivity, signal strength is just one of the many. It could be a Verizon issue definitely. It could be congestion on the network/tower in your area. It could be a problem with the host you are connecting to for downloads/speed tests, or a bad hop between you and that host. Among a list of many other things.
Your custom ROM should have little, if anything to do with your download speeds.
Side note, I don't trust the speeds that the speedtest app in the market gives if you are using that. I have been able to get better, and what I consider more accurate, results using a different options. Speakeasy's website offering isn't bad. Another option, which I do on my desktop quite often, is to download a Linux distro via torrent (which is doable on a phone). Note that with the torrent you may have to give it a few minutes to connect to seeders and get up to speed, so if you aren't on unlimited this may not be a good idea. But theoretically if you are on a well seeded distro, you would take the host issues out of the picture.
