I do agree entirely that the floor joist as presented (and for this frame as described) does seem that it could be possibly a bit too springy to me as well. Too much spring is as bad as too hard/stiff a surface to work on...

We will have to disagree on the concrete flooring Tim. I have too many friends and collegues with severe to chronic lower back and leg issues from standing and working on concrete floors...

Beside the environmental impact the entire industry has on the environment, and the less than questionable durability of modern concretes (e.g. our steel and concrete infrastructure is falling apart)...there is clear evidence of the health risks associated with standing and working on concrete. Yes one can lay matting down as many factories now do (and some areas require by law) yet just using wood solves the issue nicely, and fits a timber frame much better (in my view aesthetically.) I have seen great Lofting Floors in wood as well so that isn't really a challenge to not using wood. Even a stone paver system laid in sand is more giving than concrete is, and these too have been employed for lofting when installed neatly and for that intent.

Just a sampling of info about concrete floors...There is a great deal more out there:

Redfern and Chaffin (1995)
"...Flooring did affect workers perception of discomfort and all floor surfaces were rated better than concrete...."

Krumwiede et al., (1998)
"...investigated the effects of floor surfaces on comfort ratings in 3h of prolonged standing with 1h on each type of floor surface. Mat compressibility (ranged from 2.2 to 8.9%) was important in the comfort ratings and all surface types rated better than concrete..."