Consider a search page (say kayak.com for a good example) , with lots of options/filters. Filtering updates the results through Ajax, so without a page refresh. 

On most of those sites, after (de)selecting an option/filter a 'please wait while updating results' modal appears. Presumably to: 

* let the user know the action was successfully received
* to give the user something to do while waiting
* to disable all other actions possible clicks for a moment, ensuring a successive order of actions event taken (in contrast to mlutiple ajax-calls potentially overlapping if the user selects another filter before the previous click has returned a result.

To me this all comes down to the fact that the page needs some noticable time to update, say a second. 

I'm wondering if it would still be considered best practice to show such a notice if waiting time was reduced considerably. (Thinking ~100ms including clientside updates)

On the one hand I feel such a modal slows down the flow of the user. On the other, it may give a good indication (reflection point) for the user that the next results-page shown is the result of his/her action. 

What do you think?