When was the last time you wiped the phone clean and started from a clean slate? Probably that's what you need to do. Android devices, just like Windows PCs, get bogged down over time and you periodically need to just clean them up. So backup your precious photos and files on your sd card or somewhere else and factory reset. The S3 is a pretty old phone and it's ok for it to be overwhelmed by things; it's 3 years since it entered the market. About the battery, I wouldn't expect much from a 2100 mAh unit either. Add to the fact that performance degrades over time and usage and you won't get much from it. Probably it is time you started considering an upgrade. It is hard to know what could be the cause of delayed texts and SIM toolkit being unresponsive so first try the factory reset and give us feedback.