The first boot into MIUI 9 may take up to 10 minutes. Once you reach the setup screen, proceed as usual. To stay on MIUI 9, you must disable automatic updates: Go to Settings > About Phone > System Update. Tap the three dots in the top right corner. Select Update Settings.
Given the extreme risk, the paper recommends the following instead of a full downgrade: poco f1 downgrade to miui 9
Xiaomi requires a 7-day (168 hour) wait to unlock most devices. The first boot into MIUI 9 may take up to 10 minutes