Though some think Di2 handles the OP's question better. I currently find that my 105 5800 handled it exceptionally well and quicker by being able to drop the rear a couple or even three cogs much quicker than my Ultegra R8050 can.
But that might change, I think there is an adjustment in e-tube project that controls how fast the DR's move. I've just begun to play with that so I'll have to see if I can get it doing the required shifts on the rear as quick as they happened on my 105 equipped bike.
For the most part when dropping to the small front the rear shifts quick enough while in full syncro and my cadence and power output isn't affected. However there do seem to be certain times that for what ever reason it is an issue and I'm suddenly spinning too fast too long.