I haven't made any real speed tests yet. But loaded up Lemming880's "Boxx" and recorded a short video and checked how often the screen updates. It's update every second frame. So it runs in 25 fps. No matter how fast your Amiga is.
My game has a resolution of 320x176 (320x240 with panel). It has a lot more objects than Lemming880's game. And guess what... it runs in 12.5 fps!
... Backbone is a joke
Changing between
Chunky and Slide
scrolling in
screen setup doesn't makes no difference either.
I see now why no one else has made a shoot'em'up game with Backbone before.