Tagging user in comments takes like 2-3 seconds after filling typing the username

Currently, when I try to tag a user in comments, the dropdown menu for users appears instantly, but after writing the username, it takes so much time to load, takes like 5-6 seconds after I already wrote it to appear in the comments.
When the user writes only “@” and searches the user in the dropdown menu, it’s okay.

When the user writes almost the whole “@username”, it takes kinda a lot time.