I did a little digging on this question with my bikes. To the OP question, focus on the 2013 Felt Z85 (mid-range road bike) and the 1995 Mondonico (high-end road bike in the 90s). In brief, this project shows a new AL road bike is about 1-2 mph faster.
ANOTHER average speed thread..... sorry
Two other data points comparing these two bikes with the same rider:
For those that go
deep into bike-number-nerding... VeloViewer assigns scores to your Strava PRs; you can take those numbers to give your bikes a score. With my Felt I have a 99.83 and with my Mondonico I have a 98.87. Like the linked thread, the Felt has more miles and more segments, so this creates a slight skew to the newer bike.
Lastly, I have a solo-century route I ride a few times a year. I use both the Felt and Mondonico on these rides to keep it interesting. The Mondonico has my fastest solo century time; however, it is only 1 minute faster than the next fastest; and 3 minutes faster than average. That's a small advantage (.3% to .9%).
In summation, newer bike can help a rider be faster, but it's an incremental advantage. The best way to get faster is work on the engine.