On the parts angle, I don't think it would be that bad. Parts are already HTTP accessible from LDraw.org, although I doubt it's set-up to handle this kind of load.
An app can cache the parts using one of the several HTML5 storage options, as well as using HTTP caches. Additionally, parts average 10k each and subparts average 5k (very compressible). Include procedurally-generated subparts (a la LDView), and I would consider this reasonable downloads.
As for the speed of JavaScript, Chrome and Firefox are both JIT compiling now, so it should be comparable in speed to Java.
An app can cache the parts using one of the several HTML5 storage options, as well as using HTTP caches. Additionally, parts average 10k each and subparts average 5k (very compressible). Include procedurally-generated subparts (a la LDView), and I would consider this reasonable downloads.
As for the speed of JavaScript, Chrome and Firefox are both JIT compiling now, so it should be comparable in speed to Java.