Yes. We had a performance baseline on an iPhone 17 Pro from the day before iOS 27 shipped, and we measured it three times after. Two hours in it was 9.5% faster. Two days in it was 26.2% faster. Nothing we tested got worse at any point.
That is the answer, and you can stop here. The rest is why those two numbers are so far apart, the three days after the update when your phone will feel slower and why that is not iOS 27’s fault, and the things we did not measure — including battery, which is the one most people actually mean when they ask this question.
Corrected 13 October 2026: this article originally reported +9.5% as the settled figure. That was our two-hour measurement, taken while the phone was still re-indexing. The settled figure is +26.2%. All three measurements are now shown.
The number
| iPhone 17 Pro | Speedometer 3.1 | vs before |
|---|---|---|
| iOS 26.6.2, the day before | 43.2 | — |
| iOS 27.0, 2 hours after updating | 47.3 | +9.5% |
| iOS 27.0, 24 hours after | 52.3 | +21.1% |
| iOS 27.0, 48 hours after | 54.5 | +26.2% |
| Our noise floor for this device | 5.3% | |
The noise floor matters more than the headline. Before the update we ran the same test three times on the same phone in the same conditions and the results still varied by 5.3% between the best and worst run. That is how much this device wobbles on its own. Any change smaller than that tells you nothing.
9.5% clears 5.3%, but not by a wide margin. We would not describe this as a phone that feels faster. It is a phone that measures faster, by an amount you would have to look for.
Your phone will feel slower for about three days
This is the part that produces most of the “iOS 27 ruined my phone” posts, and it is worth understanding before you update rather than after.
When a major iOS version installs, the phone spends the next couple of days rebuilding things in the background: re-indexing for search, re-analysing your photo library, re-optimising apps. On our iPhone that window ran about 72 hours. During it the phone is warmer, the battery drains faster, and everything is a little slower — and none of that is what iOS 27 is like.
We measured inside that window three times, and you can watch it happen in the table above. Two hours after the update the phone was only 9.5% ahead of where it started. A day later it was 21.1% ahead. Two days later, 26.2%. The phone did not get faster three times over — it was busy, and it gradually stopped being busy.
This is the practical reason not to judge an update on day one. A measurement taken two hours after installing gave us barely a third of the real gain. If you update tonight and your phone feels wrong tomorrow, wait three days before forming an opinion — and if you are benchmarking it yourself, the number you take on the first evening will understate what you actually got.
Our last measurement was at 47 hours 30 minutes, still inside the window. The +26.2% is therefore a floor, not a ceiling — a measurement at 72 hours or later would likely have been higher still. We watched what the devices were actually doing during that window and measured how much of the processor it takes, which turned out to be less punishing than its reputation.
It is not just the phone
We ran the same before-and-after on everything else we own. All of it got faster, and the spread between devices is larger than the gain on any one of them.
| Device | Before | After | Change | Noise floor |
|---|---|---|---|---|
| iPad Pro 13-inch, M4 | 40.9 | 51.1 | +24.9% | 1.2% |
| MacBook Pro, M5 Pro | 55.0 | 61.8 | +12.4% | 2.7% |
| iPhone 17 Pro | 43.2 | 54.5 | +26.2% | 5.3% |
| MacBook Pro, M1 Pro (2021) | 35.65 | 38.76 | +8.7% | 2.0% |
The four-year-old MacBook gained 8.7% and the newest iPad gained 24.9%. The old machine was not left behind, which is the opposite of what people expect from a major OS release and the reason we bothered to measure the old ones at all. The full set, with each device’s own noise floor, is in our five-device write-up.
What this does not tell you
Speedometer measures how fast a browser runs web applications. It is a good proxy for the kind of work a phone does constantly, and it is reproducible, which is why we use it. It is not a measure of how fast your camera opens or how smoothly a game runs.
More importantly: we have no battery data, and battery is probably what you meant. Measuring battery drain properly on an iPhone requires a long controlled discharge, because the phone reports charge in whole percentage points — over a one-hour test a 2% drop carries an error of roughly ±50%, which is not a measurement. We chose not to publish a number we could not stand behind. If your question is “will iOS 27 hurt my battery,” this article does not answer it, and we would rather say so than guess.
Check your own phone
You can still do this if you have not updated yet, and it takes about ten minutes. The baseline is the part you cannot recover later — once you tap update, the before number is gone forever.
Safari → browserbench.org → Speedometer 3.1 → Start Test
Run 1: ____ Run 2: ____ Run 3: ____
Highest minus lowest = your noise floor- Close every other app. Turn Low Power Mode off. Unplug the charger.
- Set brightness to about half and turn off auto-brightness, so the screen draws the same power both times.
- In Safari, go to browserbench.org and run Speedometer 3.1. Run it three times, waiting two minutes between runs.
- Write down all three scores, not just the best one. The gap between your highest and lowest is your noise floor — any change after updating that is smaller than that gap means nothing.
- Update. Then wait three days before measuring again, and repeat exactly the same way.
Step four is the one people skip, and skipping it is how you end up certain that an update made your phone slower when what you actually measured was two runs of a noisy test.
So should you update?
On the evidence we have: yes, with no performance reason to wait. Our phone measured faster, every other device we own measured faster, and the oldest machine in the house gained nearly as much as the phone did.
The honest caveats are that a quarter more browser throughput is the kind of thing you measure rather than feel, that we cannot tell you anything about battery, and that this is one iPhone 17 Pro rather than a sample. If you are waiting, wait for a reason we have not tested — a specific app you depend on, or a bug report that matches your setup. “Updates make phones slower” is not that reason, at least not on this one — and if your worry is an older handset rather than a new one, we ran the same test on an iPhone 14 Pro.
Common questions
Is iOS 27 good for the iPhone 17?
On browser performance, yes. Our iPhone 17 Pro measured 26.2% faster two days after updating than it did the day before, against a 5.3% noise floor. Nothing we tested got slower.
How long should I wait before judging the update?
Three days. We measured the same phone at three points and the gain was +9.5% at two hours, +21.1% at a day, +26.2% at two days. A judgement made on the first evening sees about a third of what you actually got.
Will iOS 27 drain my battery?
We did not measure battery life and will not guess at it. What we can tell you is that for roughly 72 hours after the update the phone is re-indexing in the background, and during that window it runs warmer and drains faster. That part is temporary and is not what iOS 27 is like afterwards.
Does iOS 27 slow down older iPhones?
Not on ours. We ran the same test on an iPhone 14 Pro and it gained more than the new phone did — the full comparison is here.
What we have not tested
- Battery life, on any device. See above. We dropped it rather than publish a number with a ±50% error.
- One phone, one model. An iPhone 17 Pro at 100% battery health. An older iPhone with a degraded battery is a different machine and could behave differently.
- Browser performance only. No camera, no games, no cellular, no thermal behaviour under sustained load.
- We did not test iOS 27.1 or anything later. These numbers are iOS 26.6.2 against iOS 27.0.
If you wear an Apple Watch, check it before you update anything else: watchOS 27 dropped several older models. Here is what actually changes when your Apple Watch is left off the list.
Baseline measured 14 September 2026 on iPhone 17 Pro (MG7K4LL/A, US) running iOS 26.6.2 at 100% battery health. Updated to iOS 27.0 on 15 September 2026 at 16:33 local time and re-measured three times: 1 hour 54 minutes, 23 hours 30 minutes and 47 hours 30 minutes after that. All three sit inside the roughly 72-hour settling window, so the final figure is a floor rather than a ceiling. Speedometer 3.1 in Safari via browserbench.org, three runs two minutes apart, other apps closed, Low Power Mode off, charger disconnected, brightness fixed at 50% with auto-brightness off. Median of three runs reported. Comparison devices measured the same way on the same dates.