Flex is bad in most metals because it leads to work hardening, and they snap - not ti though. Moots YBB frames use flexing titanium chainstays to build a short travel full sus with no pivots.

I love the look of that bike - looks like a great frame for long distances without the discomfort of aluminium or whatever. I guess you're losing a bit of energy bending the frame side to side (disappears as heat eventually) but really - that much? If it *is* that much then I'm sure that redesigning and altering the frame would be crazy-uneconomic - carbon frames are a great compromise and quite cheap now.