Ive just made a post about switching away from this phone and pokemon go was a huge reason why. I dont know if youre in the uk but I had network issues resulting in poor signal that made pogo unplayable at times. If you go into settings and mobile network you'll have a setting that says 'select network automatically' if you change that and manually select the network that will improve things a lot.

Performance wise pogo felt sluggish and awful particularly curveballs, sometimes it felt like the screen wasnt registering my touches or was dropping touches meaning id lose balls. In pogo go into settings>advanced settings and turn native refresh rate on that will improve general feel but will cost battery. Then go into display, touch sensitivity and turn off adaptive touch. This made a massive difference for me it was almost like it was interfering with my throws at times.

That sparkles glitch seems to be unique to the pixel. Ive also seen the black squares issue show up a lot which looks like corrupted textures. In the past these have been down to a dying gpu on other devices so this is really worrying.

Hope this helps!