This extension is inspired by https://github.com/biji/simplenetspeed/ which has some annoying flickers existing for a long time so I decide to create my own alternative. I removed all features that I won't use.
It only shows text like ↓ 777 K/s ↑ 2.33 K/s on the right part of panel. No font size changing, no total downloaded bytes, no bits.
$ git clone https://github.com/AlynxZhou/gnome-shell-extension-net-speed.git ~/.local/share/gnome-shell/extensions/netspeed@alynx.one
or install it from https://extensions.gnome.org/extension/4478/net-speed/.
Then restart GNOME Shell and enable Net Speed from GNOME Extensions.