Can I make bash stop parsing and validating a certain alias?

My best guess is you should probably use single quotes around the alias definition.

I know that when using double quotes, shell variables are replaced with their content at the alias definition stage (like you said parsing and validating) and backticks or shell substitution like $(command).

A better explanation is in this Unix SE question!

If that doesn't help making the prompt load faster again, define a shell function instead of an alias.

edit: Don't forget to swap the cut argument to double-quotes like quixotic mentioned.

bash is interpreting your quoted string, and that interpretation executes the embedded dnf check-update command. This execution is what takes up the time during the alias definition, not the main dnf updateinfo command you're aliasing. Try a contrived example based on sleep and note how the alias itself takes 5 seconds:

alias sleep5="echo 'wake' ; `sleep 5` ; echo 'done'"

Use single-quotes to avoid the interpretation:

alias releasenotes='dnf updateinfo --refresh info `dnf check-update | cut -d "." -f 1 | xargs` | less'

Don't forget to swap the cut argument to double-quotes.