Hi,
Try starting the phone in [https://support.sonymobile.com/global-en/xperiam4aqua/faq/system-&-performance/0231018877589a7b01429399b11c00699b/#gref|safe mode] and check its’ performance.
It it works OK in safe mode then a ''downloaded app'' is the cause of the problem.
The trick is to find out which one. You’ll need to uninstall each downloaded app,one by one, and test the phone between each uninstall to check its’ performance. When it does start to perform normally then the last downloaded app that you uninstalled is the culprit.