"MaximumId + 1" to set userId is taking too long (+12k users)

Use the record ID.

You can do this by using the Users collection via API.
Add a new number parameter to hold this in your database, call it “Record ID”.

Create a user via API using a custom form and a custom action.

Add a login action following the Create User action.

Perform an update action to update the “Record ID” parameter with the Adalo internal record ID for the logged-in user.

Link home.