Casio fx-991EX vs Hp 50g speed difference

[Tugdual]

Found something weird.
Was speed testing the fx991ex vs 50g on \(\int_0^{500}e^{-x} dx \)
Results:
fx-991EX: result:1 time:6.64s
50g using ->NUM : result:1 time:46.69s

Huge difference...

Pressing EVAL on the 50g returns instantly \( - \frac {1-e^{500}}{e^{500}} \) which seems to indicate that CAS was used since this is the exact result.

I'm a bit surprised the 50g performs so poorly since I doubt the fx991ex uses any smart CAS behind the scene. Any thoughts on that?