YouTube is almost entirely C++ and Dropbox rewrote their sync engine years ago in Rust because of how ridiculously slow their Python implementation was.
No one is saying you can't make a web site using Python. Just that it is inherently a slower language that is far poorer at concurrency than many other languages.
Python is not slow for concurrency: Python has built-in cooperative concurrency that's pretty efficient, and if you care about minimum task time vs running more tasks in parallel, you need to look at multiprocess execution. In the latter case, you are generally inefficient with memory use, but if that's not an issue, you'll be effectively using your CPUs.
However, Python is slow compared to most compiled languages for simple large loops over any data structure.
A good example is how ORM libraries will fetch a million rows from the database as a list of tuples in roughly the time the db emits them at (say, a second with C-based code), yet it will take them 100x or 1000x as long to turn those rows of tuples into Python objects with pure Python code.
