By design, the platform implements 'session synchronisation' which means that for each user , the platform will only process 1 transaction at a time (for that specific user).
This is implemented to prevent 1 user from causing large instance performance issues by triggering multiple transactions concurrently.
If a user were to open 3 tabs at the same time with each one displaying a list/homepage/dashboard etc, the browser tabs will load the data on each tab one at a time. This is session syncronisation in play.
This behaviour can be seen by looking at the 'client transaction' logs - modify the list to add in the 'session wait time' column if it is not listed. Then filter out the transactions for the user that you are interested in, and sort the transactions in chronological order (Created).
This can also be identified in the localhost log files from the app nodes. If you see a user transaction has high 'session wait', then you can see as per the screen shot below, that this transaction processed really fast (0.008ms) & SQL time is 0.001ms). The key figure here is the session wait time which is 9.8 seconds - which means this transaction had to wait almost 10 seconds for the transaction prior to complete.
In scenarios like this, the users of the instance need to understand how session synchronisation works, and need to be aware that this can come across to the end user that the platform is slow, but meanwhile this is expected behaviour.