Keep using Excel or move to Use Google Sheets. These are applications that were specifically designed to handle that number of calculations quickly.
With 18,850 fields, you're asking way too much of Acrobat and PDF. You might get better performance if you don't have any calculated fields at all and use a dialog to collect the inputs, run the calculations, set the delay property to true, populate the fields with their values, then set it to false again but honestly, I still don't think you'll find the performance anywhere near that of Excel.
As background, the delay property set to true will stop Acrobat from generating field appearances every time a field changes value. Setting it to false will allow all fields to update after they have their new values set.
Thanks for replying. I agree with you and already my next step was to try Google sheets but read somewhere that it may take some time to refresh data from server to local browser depending on Internet connection speed.
Also I read in a adobe forum post that adobe reader or acrobat cannot use all cores of processor and adobe's cpu usage remains low all the time.
I understand your frustration but here's the thing... other than a piece of dedicated client side spreadsheet software, you're just not going to find anything that is optimized to perform like... a piece of dedicated client side spreadsheet software.
It's time to reset your expectations.