Ubuntu 18.04 vs. 20.04 LTS Performance Preview With Intel Xeon Scalable
One of the areas where the upgraded GNU toolchain is knowingly slower is with compile speeds due to more optimization passes and other extra work involved in compiling the software on GCC 9 in trying to make the generated binaries faster.
Numpy saw a slight improvement on Ubuntu 20.04 as did the Zstd compression speed.
The newer GNU C Library (Glibc) on Ubuntu 20.04 can also help out big time in some areas.