Signal strength won't matter if the network in your area is saturated. Try turning airplane mode on and off to see if there's any difference. If not, call Verizon and report slow network. Make sure to have some speed reports to tell them from something like Speedtest so you can quantify "slow network" with actual numbers.
I had a problem like that and they discovered that a tower nearby was reporting just fine, but was having errors. My phone would disconnect from it and switch to another tower and then switch back due to perceived network strength. That caused my data to flow extremely slowly because it would be connecting and disconnecting often.