InDesign 18.4 suddenly takes 1-3 minutes to execute a command.

Looks like a problem after updating to 18.4. 

Sometimes after the update, preferences don't import correctly - as suggested - try to trash them.

https://community.adobe.com/t5/indesign-discussions/reset-indesign-preferences-and-other-troubleshooting/m-p/11990234?search-action-id=427117166674&search-result-uid=11990234

Then, do you use GREP Styles in ParaStyle definition? 

Wher do you save it? I have sometimes saving into the dropbox such a problem.

Do you save the preview with all pages? This can also slow down the speed?

Where are your images saved? If they are saved on a different remote server, it can slow down them too.

What file types are your images?

Do you have cross references? These. an slow down too.

Do work with an INDB? It can slow it down. I open durching the project, not in the finals state, open eac h file outide the INDB.

If this is isolated to the one file, export to .idml and open that, then save as a new .indd.

Long lags can also be associated with working on files stored in the cloud whcih have to be synched constantly.

I'd recommend you start troubleshooting by restoring your InDesign preferences and caches. Here's how:

https://www.rockymountaintraining.com/adobe-indesign-rebuilding-preferences-cache/