I haven’t heard of anything specifically about that. Are you trying to run any scripts on import? Perhaps something there is failing?

I just made a story with 30,000 characters and 850 en spaces, with the same grep style as you, and it did not slow down Story Editor.
I wonder what else the problem could be? Have you tried rebuilding the document using IDML?

You are asking a question that people have asked for 25 years! Often RGB images in Photoshop look far more exciting and saturated than an image printed from InDesign. Usually it is just the conversion to CMYK. But how images get converted (using what options) makes a big difference.

justification is really an aesthetic task, so it’s hard to give suggestions. That said, I find Calibri always too “open/loose.” So I would change the settings in the paragraphs style to something like
Wordspacing 80/95/133
Letterspacing -3/-2/0
Glyph scaling 98/100/102

Andy: Right… choose Edit > Keyboard Shortcuts, then choose Panel menus from the Product Area pop-up menu, then choose Paragraph: World-Ready, and then assign a keyboard shortcut to it.

Where were you reading about merging tables?

Check out this solution, but before you try the tagged text technique, read all the comments:
https://creativepro.com/merging-two-tables-together-in-indesign.php