Truncated, I don't know whether I should love you or hate you for posting that...
Hate you, because I spent several hours cursing at the seemingly irrationality of this "phenomenon"
Or love you, because I finally figured it out (more or less), and this will probably help me bigtime in the rest of my movie
First of all I had a look at the kamikaze flight (keep the forward-button (->) pressed while flying). I found out, that the "good" version comes in 5-frames-cycles. Like this:
Press (->) in frame 190
Release (->) in frame 230 ==> big slowdown
Release (->) in frame 231 ==> enormous slowdown
Release (->) in frame 232 ==> almost no slowdown *
Release (->) in frame 233 ==> little slowdown
Release (->) in frame 234 ==> medium slowdown
Release (->) in frame 235 ==> big slowdown
Release (->) in frame 236 ==> enormous slowdown
Release (->) in frame 237 ==> almost no slowdown *
So obviously, the ones with * are those that I want. But why only "almost no" slowdown?
That's the next thing I found out: The most important part of the flight is the lift-off. Or more exactly, the passage where it goes over from rising up into floating. There you got to release the (->) button in the exact right frame, or otherwise you'll suffer from an "invisible" slowdown. The only way to notice this is by looking at the frame counter, because Mario still flies the normal way, just not as fast.
If you once have this slowdown, you can get rid of it by doing a kamikaze flight (as explained above). But, if you do it right, you don't need any more kamikaze flight, because you can't get any faster anymore and it will only cost you ~ 2 frames in the end
So, knowing this, I went back to all the levels where I started with feather and used it for flying (Donut Plains 1 doesn't count, because there I didn't have a normal "lift-off"). And I was glad to find out that I somehow always did it right "accidentally", just doing several tries and picking the best.
So the only level I had to redo was Donut Plains 4, and here I saved another 32 frames AND could do a much more entertaining flight, bumping on so many enemies (some of them above the screen) that I collect 2x 1UP in the end.
Check it out, it looks really cool now :)
Now... finally... FINALLY... I will do castle#2!!! (I was already half-way through it yesterday :S)