So, I got a Samsung J6 as a work phone. The fact that it was a "budget phone" and worked so well impressed me. So, I gave it to my wife and purcased a J6+ (dual sim) for myself. Not too sure if it was like this on start-up but boy is it slow. Most noticeably with Google Maps. Just to load it will take 10+ seconds when my wife's phone will load almost straight away.
It also has issues when switching to landscape mode and going into certain apps.
Is the software on your phone up to date? Go to Apps > Settings > About device > Software update. Also, let's try a soft reset on the phone. This will end all processes and give the phone a fresh start. To do this, press and hold the 'Power' and 'Volume Down' buttons until the phone restarts.
Let us know if this makes any difference.