The tunnel renders in about : * 1.1 frames on a stock Amiga 1200 (I must say I'm quite disappointed I just can't make it take less than 1 frame); * 0.28 frames on an Amiga 1200 + Blizzard 1230-IV with 60 ns RAM (the machine used to record the video).
I managed to get the effect to run in less than one frame also on a stock Amiga 1200. I was trying to be too smart, so I thought I'd try to dumb the code down... and it worked! The time needed to render the tunnel went from ~1.1 frames (~22 ms) to ~0.937 frames (~18.74 ms). On the other hand, unsurprisingly, the simplified code performs worse on expanded machines: on an Amiga 1200 with Blizzard 1230-IV with 60 ns RAM, it went from ~0.28 frames (~5.6 ms) to ~0.48 frames (~9.52 ms).
This video shows the effect running on a stock Amiga 1200. The second part shows visually the rendering time: the blue blackground represents the part of a frame (the whole screen + vertical blanking area) taken by the the tunnel rendering; in other words, the non-blue blackground represents the unused time of a frame.