OK, here is an updated version:
- Removed excessive enemies and replaced many with Snifits, Hammer Bros & Bullies.
- Removed the 2 Thwomps in the passage to the first star.
- Made the Thwomps easier to pass. Added a small 3x2 hole underneath the larger Thwomps.
- Route to the Green Switch is a bit simpler.
- Changed music to 'Run Through Speed Highway' from SA1.
- Made the waterfall swimmable for easier access to the pipe.
- Removed invisible blocks in first section.
- Made the second Ace Coin slightly more challenging.
- Replaced Tanooki Block with a Yoshi.
- Added 2 Leaf power-ups.
I didn't experience any slowdown so hopefully it's all good.