I also noticed since the update to Zotero 6 that this feature seems to hang a bit. Previously the library was very responsive and would instantaneously jump down (even when I had thousands of entries in a single collection). In 6, there is a delay of a few letters (even at a casual rate of typing) and then the library catches up. The effect might look a little "smoother" now (rather than rapidly jumping as you type), but it's less responsive and doesn't give the immediate feedback of whether you typed the right letters and/or whether there are (approximate) matches in the library. It's a minor inconvenience, but I preferred the previous version.