The rest would be optimization while keeping the timing sidechannel constraint in mind, hard to say what the performance possibilities are. For example not all computations have externally observable side effects, so those parts could be executed conventionally if the runtime could guarantee it. Or the program-visible clock APIs might be keeping virtual time that makes it seem from timing POV that operations are slower than they are, combined with network API checkpoints that halt execution until virtual time catches up with real time. Etc. Seems like a interesting research area.