For some clients, opening the 901 application may take up to a minute. Once opened, the performance improves and changing filters or drawing another due date takes a few seconds (as expected).
The list should generally take between 5 and 20 seconds to open fully.
This has been resolved in Tax NZ 2020 Q3 hotfixes available from the TaxNZ Downloads page
It was found to affect clients with large numbers of 901 notes.