Thanks for the responses. To answer the question - I have a Planet-X Nanolight carbon frame so it is a monocoque type frame.

One observation that I had for double-tap is when the actual shift occurs.

When downshifting to a larger cog (increasing cable tension) the actual shift occurs after the first click as you continue pressure on the shift lever to the second click.

When up shifting to smaller cog (reducing cable tension) the actual shift occurs after the click as you release the shift lever.

All this to say that for up shifts, any resistance in the cable, shift lever or derailleur will slow the shift. Perhaps I have some resistance somewhere in the path that is slowing the shift a little. As I said, functionally the shifting seems OK as it doesn't skip or miss a shift. It just feels like it could be a little more crisp, especially on up shifts.

Downshifts appear to be more affected by how much pressure I put on the shift lever. Presumably, because cable tension has to be increased to actually make the shift happen.

Perhaps I'll look into replacing the cable and housing (I do have almost 3500 miles on the bike) over the winter. One other idea I have read about is to make sure that the cable loop that goes into the derailleur is long enough so that the cable can move easily. I think the video I saw said it should be 36 cm.