ngen.exe doesn't make the dll run any quicker, it can make the loading quicker though by removing the need to just-in-time compile methods when they are first used. You might not even notice a difference in a lot of cases between an ngened assembly and a normal assembly.
The precompiled assemblies are also very machine specific (cpu, os, permission and framework version) and would need to be redone if any of several things change otherwise it will just fall back to doing JIT compiles anyway.