Array.from makes an extra function call each iteration and function calls are very expensive in interpreted languages. Compilers or interpreters sometimes optimizes it away, but in this case it didn't.

Remember this: Compiler optimizations are very unreliable, small innocuous changes in code can get huge effects on performance.

Edit: Just looked at the code in the link, and Array.from is not the only difference, the other code uses string concatenation instead of array concatenation. Appending to a string in Javascript is typically a lot faster than appending to an array.