Is the long calculation completely decoupled from the application? In other words, does it depend on changes made in the application directly? It looks like the callbacks are consuming output files from it, so I wonder if this long calculation could run in a different process.
As a side note, you could take a look on the long_callback functionality pre-released in Dash 2.0:
https://dash.plotly.com/long-callbacks
There is also a feedback thread that you might find interesting to follow.