I have a 5S and recently had to upgrade so that a vendor would provide technical support. They stated they only provide support for their application running on

I have a 5S and recently had to upgrade so that a vendor would provide technical support. They stated they only provide support for their application running on the latest version of iOS that Apple supports for your device. I had held off and finally upgraded as the app is fairly important to me and I wanted to get the issue resolved. Now input is 10+ seconds lagged, everything is significantly slower and apps frequently run out of memory and I end up at the home screen. Unfortunately the iOS upgrade didn’t fix the issue and I’m still dealing with the vendor.