How to not block callbacks between users/sessions?

I noticed, when I open Dash in two windows and I execute a long lasting callback in one window, the other window stops executing until all callbacks in from the first window are finished. I wonder, if there is maybe something wrong with my setup? I am using waitress to serve the application. Is there something that would allow parallel execution without blocking such executions from a different windows?

So, when one user executes a long-running callback the app is blocked for other users.

This is the command that I am using to start the app:

waitress-serve --port 8000 --threads=16 --call 'flaskapp:create_app'