Thats what every company do they provide the update to oldest phone so that it become slow because the processor is not that much powerful as it was earlier so it takes more time to do some tasks which results you to buy a new smartphone that's why they provide update to older phones otherwise you know that your phone won't be taking that much time earlier to do same task that's how they make money.
My j6 is even faster than before. I am having a great experience. Yes, the front camera performance is a bit downgraded but, that's ok for me.
I am shearing my update steps. 1. took backup of all data using smart switch 2. download the update file of around 3.1 gb via smart switch. 3. updated my phone. 4. factory reset my phone. 5. fresh start.