Will IOS 27 make older iPhones slower?

Probably on some older iPhones, yes — but not always, and it will depend a lot on the specific model, battery health, and what features iOS 27 adds.

In general, major iOS updates can make older devices feel slower for a few reasons:

• Heavier software demands: New versions often add features, background tasks, and visual effects that can be harder for aging hardware to handle.
• Battery health: If an iPhone’s battery has degraded, iOS may reduce performance in some situations to avoid sudden shutdowns.
• Temporary post-update slowdowns: Right after a big update, the phone may feel slower for a short time while it finishes background work like indexing and syncing.

So the safest expectation is: older iPhones may feel slower after iOS 27, but not every older iPhone will be affected the same way.

If you’re worried about a future update, check your battery health, look for reports from users with the same iPhone model, and wait a bit after release before installing if performance is a priority.