TgCrypto is a replacement for the much slower PyAES and implements the crypto algorithms Telegram requires, namely AES-IGE 256 bit (used in MTProto v2.0) and AES-CTR 256 bit (used for CDN encrypted files).
$ pip3 install --upgrade tgcrypto
Being a C extension for Python, TgCrypto is an optional but highly recommended dependency; when TgCrypto is not detected in your system, Pyrogram will automatically fall back to PyAES and will show you a warning.
The reason about being an optional package is that TgCrypto requires some extra system tools in order to be compiled. The errors you receive when trying to install TgCrypto are system dependent, but also descriptive enough to understand what you should do next:
Windows: Install Visual C++ 2015 Build Tools.
macOS: A pop-up will automatically ask you to install the command line developer tools.
Linux: Install a proper C compiler (
clang) and the Python header files (
Termux (Android): Install
Although TgCrypto is intended for Pyrogram, it is shipped as a standalone package and can thus be used for other Python projects too.