Why is my iPhone Wi-Fi so slow after updating to iOS 18 or 18.0.1?

Since updating to iOS 18 or 18.0.1, my iPhone has been experiencing WiFi instability. Connections frequently drop, speeds are significantly slower, and websites often fail to load, even when the device indicates a strong WiFi connection. This is particularly noticeable when streaming videos.

[Re-Titled by Moderator]