I'm sorry you experience difficulties with Nisus Writer Pro.
We are, however, not aware of any specific reason why your typing
should be slow. Here are some things you can do to encourage a speed-up:
A.
Write in Draft view rather than Page View.
B.
Turn off Check spelling as you type.
C.
Quit all other applications.
Those are free. The following cost money:
A.
Get a faster Mac.
B.
Get more RAM.
Each new version of the OS from Apple utilizes more resources and
requires one of the faster machines that they make. It's like a game.
Apple wants us to buy new computers to use the new software.