Dendwaler, I visited your Age yesterday and I enjoyed it immensely.
This is excellent work in all respects.
Parts of it remind me of Riven, like the huge metal machine in the water near the tower, and the color of the water.
I liked almost everything, especially: models, textures, lighting where it is finished, sounds, music, journey cloths (so THAT's what your avatar is!), all the little details that must have taken you weeks to get done: marshes, flowers, cacti, fountains, etc.
I liked:
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I had several "Wow!" moments. The concert hall was one of them. I wish it was possible to click on the instruments and hear them.
I never thought finding 7 journey cloths could be so difficult, even on fly mode!. I loaded the Age in Prpshop to make sure there were indeed 7 of them.
I didn't like:
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The ship. I liked the idea of a circumnavigating ship, but didn't like the ship itself. It didn't seem a Spanish galleon fits into the Age's theme.
I didn't like the pipes for the same reason, and they don't seem to have a purpose, and it was nearly impossible to get on them because of the current.
The size. Exploring is fun, but running around for hours is not. The island is much too big for what you do with it. I switched to fly mode often.
The moai. The texture and the specularity are weird, and they don't fit with the concert hall.
The ladder!!! Aaargh, ladders in Uru are an abomination. At least let us open the door from the outside. I went back inside the tower 3 times to check for that last elusive journey cloth, thank god for fly mode.
Like most people, I had performance problems even with my powerful gaming computer.
Graphic problems: my video driver kept crashing, sometimes every 20 seconds. This happens on a lot of fan Ages, but this was the worst case yet. The posts above give plenty of good suggestions for fixing this.
Performance problems: these happen to me very rarely.
This is most likely caused by collision detection. Like you said, you need to add simple colliders and remove collision detection from your visible geometry. This has to be done everywhere, and then add only colliders you need. For example, the turtle shell has collision detection on the outside, which is not needed.
Also you could simplify some objects. The spiral stairway (drill bit) at the start is made of hundreds of copies of the same mesh, which means thousands of unneeded vertices and faces having to be rendered, and on which collision detection is set. This slows down a computer, and makes going up and down the stairs awkward. What you need is an invisible spiral collider ramp, that does not have to follow the exact shape of the stairs. Same goes for the stairs in the library at the end.
As an example, see this collider I made for the library stairs in Aegura. I just duplicated Cyan's existing collider, but I could have removed half of the faces and no one would notice.
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- stairway.jpg (139.26 KiB) Viewed 5862 times
Also for the stairs leading up to the stage in the concert hall, you only need a simple ramp. As an example, see this collider I made for the palace courtyard in Aegura. Notice the ramps at lower left and upper right.
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- NewFloor3.png (18.6 KiB) Viewed 5862 times
. The music and sound makes it even more enjoyable!
. That's using a Core 2 Duo, 2GiB of RAM and an NVidia GeForce 8500 GT with 512MiB of RAM and Windows 7. All other Uru ages, including Tvel, run fine. And this is an age that I'd really like to play 
). It's just using visregions.