The Test Page Runs Really Slowly And Stutters On My Phone

Your instinct is right and it is the most important part of this: when the page is struggling, the readings are not trustworthy. A stuttering render means frames are being skipped, and a skipped frame means a gap in the polling, which shows up as values that jump. So do not draw any conclusions about your sticks from a session that stutters, because you would be measuring the page rather than the controller.

A four year old phone is well within what this should handle, so something is taking more resources than it should. The usual causes on mobile are other browser tabs still live in the background, a browser that has been open for days accumulating memory, and battery saver mode, which throttles the processor hard and is the single biggest cause of this specific symptom. Phones also throttle aggressively once they get warm, which creates a loop: the page runs hot, the phone throttles, the page runs worse.

The practical fix is to give the test less to do. Running a single focused test rather than the full tester makes a large difference, because the full view renders the whole controller and every axis continuously. If you want to check your sticks, use the stick drift test or the deadzone test on their own. Test in bursts of twenty or thirty seconds rather than leaving it running, which keeps the phone cool and the readings clean.

If after all that it still stutters, tell us your phone model in a reply. We do test on older hardware and if a specific device is struggling more than it should, that is something we would want to fix at our end rather than yours.