You might need to upload a workbook if you can't get this working. But here's what you want to do.
DATEDIFF('second', LOOKUP(ATTR([Created]),-1) , ATTR([Created]) )
this will tell you the difference in seconds between the current row and the prior row. This is a table calc, and you will want to set your table calc setting to restart every [user_id]. (You don't want the calc comparing the first row of a user ID with the last row of the prior userID.)
Then you want to do a window_average of those values, also restarting every user_id.
You'll end up with each user_id's average value on each row within that user_id. You'll then want to filter to display only the first row of each user_id. that will give you the final value you're looking for.