iphone 6 Plus slow for months

Hello, I have a iphone 6 Plus brought new from the apple store on release and it has become so slow its almost unusable now. This began since late 2017 prior to that it was fast and responsive. opening anything and progressing within apps for mail etc results in notice lag up to seconds. I have only had the camera replaced at an apple store (as part of your fault advisement) otherwise it is undamaged and untouched by others. According to your technical support at your store here in Melbourne this is a result of software updates. The only option I have, according to your staff, is to "jailbreak" the phone which could render it completely useless. As its almost so slow its useless now this option has some appeal however I would like to have my previous fast response and performance restored to my phone. What should I do, and importantly - what can you do to help if this problem is indeed a result of the software updates you have rolled out?