I change from a 22 pounds Bianchi (w/ clip-on, size 61) to a Cervélo Soloist Carbon (size 58, with clip-on, weight??) ? 30 seconds???
Weight won’t mean much. The size depends upon which one fits you best. The only thing that really helps between the two bikes is the Soloist aero tubing. 30 seconds sounds like a reasonable guess.
My guess, less than 10 seconds. You could even be slower if it is a flat straight TT (for some very stupid technical reason only I believe). Weight has little to do with speed in a time-trial. Check it out analyticcycling.com. At their default values (except changing it to a flat course) reducing the weight a full kg from 75 to 74 only increased the speed .01 m/s from 11.23 to 11.24 m/s.
Frank
You may actually save quite a bit of time. The Bianchi is your normal road bike? With a smaller frame you can raise the seat up, create a larger drop from the saddle to the bars, and get a better position. If the head tube on the Cervelo is shorter, that’s a bigger bonus. You may also want to play with crank length to allow you to get that higher seat position/bigger drop.