Folks, it's that the external USB hard drives are spun down to save power, and they take several seconds to spin back up and be mounted by MacOS.
Rebuilding your index won't help, as @morgankrane has proved.
Until Apple FIXES THIS BUG*, there are two workarounds: (1) unplug your external drives unless you are actively using them, or (2) replace them with external SSD drives that will not have to spin down, and therefore will avoid the cause of this lag.
Vexedly -- Peter
*Apple, Spotlight should go ahead and return index hits immediately, and if the user clicks on one that links to an unmounted drive, then it can take the delay. I want my immediate calculation functionality back!!