Spent a lot of time investigating and testing: went to other places and connected the iPad to other routers, used the iPhone as mobile access point, reset and restarted everything several times. Nothing worked.
Downloaded Wi Fi Explorer and checked my airspace. The 2.4G band was pretty crowded, but the 5G band wasn't as much, and not to the point to inhibit my iPad's wifi almost completely:
Tested almost daily and performance kept being terrible:
I left my iPad clean and stopped using it completely except for testing wifi connection stability and internet performance.
iPadOS 18.1.1 was released on the 19th and I installed it a few days later. Didn't pay it any mind, but when I opened Safari to check its release notes, I immediately noticed webpages were snappy once again. Ran Infuse streaming speed tests and was relieved to see the graph being back to normal:
After that, I restarted and reset everything again, made changes to wifi channels (as I had tested doing that while monitoring my wifi signal quality with Wi Fi Explorer), and finally restored my iPad from iCloud backup to see if it kept working right. It remains normal.
I'll keep monitoring it closely, but I speculate that iPadOS 18.0 (or 18.0.1 or 18.1) borked wifi and 18.1.1 fixed it.
