Why Verizon 4G LTE is so slow?

Coverage depends where you live.  Without knowing where you are, any comments about someone having or not having fast speeds is meaningless for your location.

Verizon tends to have more consistent\stable coverage and better building penetration because of the frequencies they use\own.  Speed will more be dependant on how close you are to a cell tower, how upgraded the specific tower is and your phone's capabilities. 

  If you want speeds that clock over 20mbs consistently you will need a phone with gigabit LTE capability (4×4 MIMO) which is a feature only in the higher end phones less than 2 years old.  These phones have the capability to transmit on enough frequencies simultaneously to overcome a lot of the interference that would slowdown other phones.  

Regardless while I do find it fun to try to hit max speeds on my cell phone with speed tests, all you realistically need is a stable 12mb connection to literally do anything almost instantly on a smart phone since nothing really uses much more bandwidth.  What most people think is slow has to do with stability\latency issues rather than max speed. 

Verizon has purposefully decreased the speed of their 4G LTE from what was 25 to 30 mags all the way down to like 12 or 15. Because they want to give the 5G extra bandwidth and encourage you to purchase a 5G device. It's another technology scam my Verizon cell phone was running around 20 to 25 Meg's and now it's been decreased to like 12 or 15 if I'm lucky!

There are too many factors that affect your mobile data speeds, such as how far you are from the tower, how many people are on that tower, etc. That's one of the reasons why I don't recommend using mobile data as your main source of Internet. The speeds and service can be slow and inconsistent.