Problems I have seen.
Noting that budget phone not a flagship phone, is mostly in play. 
Still loaded with irrelevant apps, many that can't be removed.
Sucking what little life there is out of the UI. 

1/ Touchscreen operation that can be finicky.

   The hit to confidence just makes things worse as 'long hard' presses cause more problems.
   Even swipe to answer may become fiddly if you seem to have too small a finger, the wrong knack.
   Juggling holding phone right and overcompensating as your confidence is lost.

2/ Screen brightness, contrast and definition.  

  Often you are looking at phone from a not ideal distance angle, lighting conditions, and eyesight. 
  
  Ui may assume you will not linger, slow reactions because you are deciphering screen.
  If touchscreen is not that great (1) this feeds back. 

Small timid finger touches may just make this all worse, maybe some people have a different electric field ?

If they had a phone with great Touch and Screen with crap cleaned out then results maybe quite different.