Perhaps we should ask Neil__j. He has a rigid KHS and a custom built modern Ti framed bike. The Ti framed bikes geometry and design was based on his KHS with the addition of sus forks and disk brakes. He is a fast rider so maybe he can tell us which is quicker.
A little late to this and possibly not that relevant in terms of modern vs. retro, but I'd say the Ti is a bit faster but that's all due to the 80mm boingy forks. As the frames are fundamentally the same in terms of geometry it's the frame tubing/material and components that will make any difference. Both are about the same weight but the front/rear balance is different. Both 8 speed with thumbies but the Ti has a compact chainset so lower overall gearing. I typically use the same tyres at the same pressures on both.
I've used both in the last month on a (flat East Anglian) local 22 mile loop and would say it's the forks that make the most difference and only in the rougher sections, so a bit of suspension on the KHS would probably make them pretty equal. In a race like Mayhem I'm sure I'd be faster for longer on the Ti, but wouldn't get the same satisfaction of passing people both up and down hills on modern bikes as I do on the KHS.
A proper modern bike with slack geometry, massive forks and wide bars would be a whole different thing.
1993 KHS Montana Pro - rigid
custom Ti - hardtail (KHS copy)East Anglia area group