Video here comparing two road bikes, one a heavy cheap road bike with a limited 12-25 cassette but at least it is freehub based. It's groupset is mainly Claris. The bike cost £200. Another bike Boardman branded which was about £2000 but has an equivalent spec to many of the major brand bikes up to £4k in the UK. So quite extreme difference and yet the cheap bike averaged 33.3km/h vs 35.4km/h for the expensive bike. That's a 6% difference in speed, I'm sure some will argue that difference is huge and others will be disappointed by the difference between such extreme bikes its really down to your viewpoint. I think the same Muddyfox bike with perhaps a 11-34T cassette and maybe a CF fork would achieve over 34km/h but that is a guess by me. I've seen the Giant Contend Claris which is sub 10kg and sold for less than £400 at times in the UK (not now of course) would probably achieve easily over 34km/h. 17mph is about 27km/h. Ultimately the real speed difference is dictated by the rider not the bike as the speed difference between riders is huge and the speed difference between bikes is very small but if you are a competitive cyclist who wants to win a 1 second gain over 20 miles could be the difference between winning and losing.