I too have been disappointed in the performance after the upgrade to 50/25 Quantum Service. However I offer a few observations about what is going on. Testing for speed within the Verizon Network reveals the connection is really is delivering the rated speed (and then some). However speed tests on various servers that are located outside the Verizon Network invariably produce disappointing results.
Unless you have a fairly recent version of the FiOS router, on wireless you are limited to 802.11g speeds, which in theory are up to 54mbps. That assumes a perfect world with no interference. Unless you happen to have an anechoic chamber to run your network in, you aren't going to get close to 54mbps. Real world generally tops out at around 20mbps for 802.11g. Dual Band 802.11n can go much faster, but you need to have 802.11n devices, and no 802.11b/g devices active on the network. If you want 802.11n wireless speeds, you can try to convinced Verizon to swap your router for one that supports 802.11n, you can buy a new router from Verizon (~$80 IIRC), or you can just buy a 802.11n wireless access point, and connect it up to your existing Verizon supplied router (don't forget to disable wireless on the Verizon supplied router).
My own belief is that Verizon has sold a lot of upgrades to Quantum Speeds, so the aggregate demand for bandwidth has gone up considerably. This does not appear to have been accompanied by a corresponding increase in available bandwidth upstream. I.E.. We are being constrained by Verizon's connections to the 'outside' world. Hopefully at some point Verizon will add the necessary bandwidth to enable the customers to actually utilize all of the connection speed they have paid for.
