Bad RAM performance on iOS 26.2.

Who did you buy the phone from? When you buy a used iPhone from someone other than Apple or an Apple Authorized Seller, there's no telling what you got. If you're not pleased with the purchase and you can return it, that's exactly what I'd do. If you want a used iPhone, buy one from the Apple Refurbished Store, where it will come with a new screen and battery (both authentic Apple parts) and a one year warranty.

As it stands, you will need to invest about $100 to replace the battery in your phone in just a few months. The battery needs to be replaced when its health dips to 80%.

Certified Refurbished Products - Apple

Vojtech_Sorm wrote:

Got it from swappie. I do think that the iPhone is genuine, and don’t really want to go through the refund process unless it’s necessary. My old phone stopped working, and the week or so before the refund is processed and I’d received a new one from apple would be kinda difficult. Maybe it’d take even longer because it’s Christmas.

That's up to you. I wouldn't have bought the phone as I am a firm believer in ONLY buying Apple products from Apple or Apple Authorized sellers. We can't possibly know what condition the phone is inside. You know nothing about its history. Since you seem concerned about it, however, I'd return it if it was me. Or, if you want, you can schedule an appointment at your Apple Store Genius Bar and have Apple test it --> Genius Bar Reservation and Apple Support Options - Apple

There is no warranty on the phone, so if there's an issue, you'll have to pay for any repairs, plus as mentioned, you will soon be paying Apple $100 to replace the battery.

Vojtech_Sorm wrote:

My phone cannot remember the last 5 apps opened.

Being unable to keep four apps running in the background would not be normal unless some of your apps were extremely RAM-hungry. (E.g., games). Especially on an iPhone 15 Pro Max which has 8 GB of RAM.

With extreme RAM hogs, it might be a different story.

Press an iPhone hard enough for RAM, and iOS will go around to all of the apps, "asking" them to release RAM. ("Nice RAM that you have there. It would be a shame if anything happened to it."). If the apps won't voluntarily return enough RAM, iOS will then pick some to be victims. It will kill (crash) them, letting it take all of their RAM.

And how much free storage do you have remaining at Settings > General > iPhone Storage?