Javascript is plenty fast, but only if it has enough time to JIT the code and can keep it JITed in memory. For long-running backed services, it's plenty fast, for browser things, it's the only option, but for the command line, it adds needless startup time.

Agents make this even worse because they don't have a "sense of time", so if they accidentally do something which causes startup time to increase dramatically, they won't feel it like a human dev would, and won't immediately start optimizing. Unless you have some specific benchmarks in CI that fail any PR which makes the code too slow, agents will just make things slower and slower.