Windows 11 has a higher RAM requirements than Windows 10. It requires to have at least 4 GB of RAM. PC with 4 GB RAM will run Windows 11, but it will work slower on more intensive use.
The PC could work slower even when you will have a few windows or tabs opened, because Windows 11 uses around 3 GB or RAM on idle. When available RAM is fully used, Windows will start using virtual memory (saving and reading data from the disk), which is much slower than RAM.
Because of that I recommend to have at least 8 GB RAM for Windows 11.