A phone that felt instant two years ago runs the same hardware today. What changed is everything around it: apps grew, the operating system added features, and web pages became heavier. The device did not get slower so much as the work got bigger.
Storage is the second factor. A nearly full phone has no room for the temporary files the system relies on, and everything from opening the camera to installing updates slows noticeably.
Battery health plays a quieter role. As a battery ages it cannot deliver short bursts of power cleanly, so the system limits peak performance to avoid sudden shutdowns. This is deliberate protection, not sabotage, and it reverses with a battery replacement.
Finally, background activity accumulates. Years of installed apps refreshing, syncing and sending notifications adds up to real work happening when the screen is off.