How about one that can do multi-term queries at a decent speed while at a large scale while still giving good results?

I assume this should increase with the number of users? no?

Beware the delta between promises and reality.

I started out thinking "theory and reality", but, even in theory, adding a ton of very slow, latent, and unreliable data sources to your query doesn't produce a fast result.

No. I don't believe Yacy will reach exceptable performance as it scales.

I encourage people interested to look at "Peer-to-Peer Information Retrieval: An Overview" in order to get a better understanding on what technical challenges distributed search engines like Yacy have yet to overcome and the paths researchers are taking to overcome them.