Funny you mention this. I am in my taper for StG now and have vowed to put the Shiv up and just do prep on it until race day. In the meantime, I'm riding my 2015 Cannondale Synapse for short maintenance/training rides. They're both the same age and in comparable shape. I do have a set of Profile Designs aero bars on the Synapse so I can get kind of low on it, and considering my back and neck, I'm really not a whole lot lower on the Shiv. But I have noticed the last couple times I rode the Synapse, I can just feel it's not as fast for the same amount of effort. I don't have a power meter (but I do have a bike computer) so I'm just ball-parking it, but I would say that at the same power level, I can hold better than a 1mph faster pace on the Shiv than the Synapse. Might even be as much as 2mph. I'm really not sure where all that is coming from. I suspect it's a combination of factors. I'm more comfortable on the Shiv, it has shorter cranks and I'm running tubeless Reynolds carbon wheels. There might be a couple other factors, but whatever they are, I can just feel it.

I am kind of drawing the line at "upgrading" to electronic shifting and disc brakes. But if I could find another frame that allowed me to get an even more aggressive position, I would consider that. I really prefer a steep seat tube angle and I've struggled to find a set up that is comfortable and I can put power to the pedals.