trying to figure out why i have this device

Edge 820

1.) device takes FOREVER to calculate a route.... often 20 minutes!!??
2.) cannot effectively create a route on the device. too awkward. really needs to be done on desktop and then sync'd with device.
3.) my biggest issue.  if i go off route for any reason while riding, it's basically a total disaster.  it starts to recalculate (again, taking FOREVER)... taking me back to the BEGINNING of my route???

i think about doing all of these things on my phone via Google maps or Waze.  takes about 3-5 seconds to create a route....even in a dense area like central London and even for very long journey.  i can do it on my phone effectively.  if i decide to make a detour while travelling, it recalculates on the fly and keeps me on my way to my ultimate destination in a very sensible way.  it has a few quirks, but it basically just works.

my phone is too delicate to be on the front of my bike in bad weather...and the battery wouldn't last on a full day ride.  so i got this Edge 820.  why...in 2020... is my NAVIGATION device unable to NAVIGATE?    we know from other navigation methods that this is all very doable...i guess just not by the world's leading brand in navigation devices??  really scratching my head here.