iPhone 15 Pro gets ~half the Wi-Fi speed of my iPhone 16 and iPad mini 7 — issue followed a backup restore from my 12 Pro Max

TL;DR: My iPhone 15 Pro consistently pulls roughly half the download speed of my iPhone 16 and iPad mini 7 on the same network. The iPad mini 7 has the same A17 Pro chip and same Wi-Fi 6E radio as the 15 Pro, so it's not a chipset generation thing. My old iPhone 12 Pro Max had the identical problem, and the 15 Pro was restored from its backup. A Personal Hotspot test suggests the phone, not the router — but I'm stuck.

Setup

• Router: TP-Link Archer AX21 (AX1800, Wi-Fi 6)
• Slow device: iPhone 15 Pro (bought yesterday, restored from iPhone 12 Pro Max backup)
• Fast devices: iPhone 16, iPad mini 7
• Note: iPad mini 7 has the same A17 Pro SoC and same Wi-Fi 6E radio as the 15 Pro, and it's fast. So "newer chipset is just better" doesn't explain it.

The numbers

Speedtest results over one evening:

Solo tests on the 15 Pro land around 320 Mbps. The download speed swings really wide, it could go from 30 to 320 then down to 180. While the 16 and the iPad mini consistently hit 500.

Every test also shows heavy bufferbloat — idle ping sits at 13–15 ms, but jumps to 300–900 ms during the download phase, with jitter in the 80–99 ms range.

What I've already tried (none of it stuck)

On the phone: – Private Wi-Fi Address → Off, and → Fixed. This temporarily fixed it once (jumped to 500), then degraded again within a day. Retrying the same method doesn't work. – Forget network + reboot + rejoin – Checked HTTP Proxy (off) and DNS (automatic) – Limit IP Address Tracking → off – iCloud Private Relay → off – Low Power Mode → off, phone plugged in – Checked Developer Mode → Network Link Conditioner is off – All speed tests were done with the VPN off – Sidestore and some sideloaded apps, but the iPad mini has the same apps.

On the router: – Checked the client list — no per-device QoS, bandwidth limit, or MAC filter rule on the 15 Pro. Its settings look identical to every other client. – All three devices are on the 5GHz network with whatever smart connection feature on the router off.

The hotspot test

Turned on Personal Hotspot on the iPhone 16 and connected the 15 Pro to it, taking the router out of the equation entirely:

• iPhone 16 (on its own cellular): ~40 Mbps
• iPhone 15 Pro (via the hotspot): ~20 Mbps

Same ~2x gap, with no router involved. That points at the phone. (Though I realize those numbers are cellular-limited and could just be cell conditions fluctuating between the two tests.)

The part that confuses me most

My iPhone 12 Pro Max had this exact same problem, at roughly the same speeds. I restored the 15 Pro from the 12 Pro Max's backup and the problem came with it. So I did a full reset on the 15 Pro, then the wifi speed was fixed, but then returns to the unstable state just 20 mins later.

Any ideas appreciated. This problem has been driving me crazy.