This is a problem that has existed before this version. For me, it's not a big issue, but I've noticed that for other users, especially when installing Thunderbird on some of my clients instead of Outlook, they always raise this question during searches.
In the Quick Filter, it would be very important to have some indication that the search is in progress. When searching for something in a folder with a large volume of emails, sometimes we are left unsure if it has found something or if it is still searching.
Therefore, something as simple as a message or an icon right after the 'Filter messages by:' options would be sufficient.