After a battery replacement, your iPhone's performance may improve, especially if the old battery had experienced significant wear and tear. Over time, Lithium-ion batteries degrade unavoidably and eventually lose their ability to hold a charge. If you're experiencing slow performance or unexpected shutdowns, a new battery can sometimes resolve these issues. However, it's important to note that other factors can also impact your iPhone's performance, such as software updates or hardware damage. If you're still experiencing issues after a battery replacement, it might be worth contacting Apple Support for further assistance.