A 33 MHz 486 CPU (the recommended spec for Second Reality to run smoothly) is MUCH more powerful than the 7 MHz 68000 of the A500... It's not just nearly 5 times faster, the 486 is way more cycle efficient, not to mention having a nice byte-addressable bitmap screen to draw into (vs 2-6 separate bitplanes).
So yeah you forgot how underpowered OCS Amigas are :)
The Amiga hardware doesn't really help much with 3D effects, which the demo is loaded with. Audio is a big help for a 68000 but a 486 can mix 8 PCM channels and not even notice.
So yeah you forgot how underpowered OCS Amigas are :)