Why do phones and computers get slower over time?

The dominant cause is that software gets more demanding while the hardware does not change. Operating system updates, app updates, and websites all expand to use the capabilities of current devices, so a machine that was comfortable at launch is running heavier code years later. Storage contributes independently: file systems slow noticeably as drives approach capacity, and background services, sync agents, and startup items accumulate over years of installs. Battery aging matters on phones specifically, since a degraded battery cannot deliver peak current, and thermal issues from dust buildup cause processors to throttle in laptops and desktops.

The conspiracy version has one real data point and a lot of overreach. Apple did throttle performance on iPhones with degraded batteries, confirmed in 2017, to prevent unexpected shutdowns, and was fined in multiple countries for not disclosing it rather than for the throttling itself, which was arguably a reasonable engineering tradeoff. That is the documented case, and no comparable evidence supports a broad practice of deliberate slowdown across manufacturers. Practical fixes in order of return: replace an aging phone battery, which is cheap and often transformative, free up storage, disable unnecessary startup and background apps, and on an older computer, add RAM or swap a mechanical hard drive for an SSD, which is the single largest real-world speed improvement available for the money.