You can't really tell for a Web App whether it'll be faster in JS or Python, but you can definitely expect a Computer Vision application with lots of heavy number crunching to be a lot faster in C++ than in Python. We have actually also made comparisons, and even if you use things like numpy and Python bindings for OpenCV, you won't reach the speed that a C++ application achieves easily without optimization.

For a lot of applications there are pretty straightforward calculations to meet a desired frame rate.

Also I've seen UX research that established some guidelines for how long something can take while still feeling responsive allowing a user to remain focused.

Knowing you can achieve the required/desired performance in any given language is mostly a matter of experience solving similar problems.