Updated: Jan 22, 2020
It seems that as the options and features in InDesign evolve, importing tagged text files gets slower and slower.
I suppose this is because there is more to check, and more complex layout decisions for InDesign to make. It might not be a problem when the text being imported isn't too big, but we frequently create documents of 100+ pages and it can take AGES to import them sometimes!
We've figured out a way to speed things up. This is what we do here at CatBase:
Create a new InDesign document with automatic text frames.
Import your tagged text file into that.
When it's done, select all, Copy, and Paste it into your InDesign template.
I did a comparison test with a 1Mb tagged text file, which produced 160 InDesign pages (using InDesign CC 2017). And the results were ...
Importing the file directly into the InDesign template: 43 minutes. Zzzzzzzzz
Using the method described above: 17 minutes
A couple of tips ...
Create a fresh document each time - don't just delete the text and re-import the file into the same blank document.
Sometimes the stylesheets get slightly confused and do not format the text exactly right in the template. This is easily remedied: Use Find and Replace to find any text in a certain style, and Replace it with the same style. That'll fix them.