That goes against general experience. Green safety chains cut a tiny bit less well than yellow non safety chains (tiny, because Stihl executes safety chains very well). The difference between Stihl semi chisel and chisel chain is very small also. Maybe he changed some other variable, i.e., dull vs. sharp? Maybe his definition of "hot knife through butter" differs from my impression. Maybe his saw does run a bit better on the slightly less aggressive green safety chain on a 20 inch bar?
A longer bar will cut a bit slower than a shorter bar (in general, same engine) because there is more chain (drag) for that engine to handle.
Again: 4 variable chains we are comparing
green chain comes in semi chisel and chisel
yellow chain comes in semi chisel and chisel