Yes, some delay issue had started manifesting already in the old version, but I’m fairly sure that is coincidental, and that there is a new or additional delay in the new UI.
In the old UI, the Vault tab only showed favorited items and items that did not have an assigned folder, it did not show the entire vault (“All Items”). Interestingly, if the number of items with no folder exceeded 100 items, then the items were no longer displayed in the Vault view, but were instead rendered as a folder named “No Folder”. In a Reddit comment, @kspearrin indicated that this behavior was implemented due to concerns regarding the “performance of loading the entire vault on the screen”.
Now, in the new UI, there no longer is any safeguard to prevent loading of >100 vault items at once, so it is no surprise that users who have a large number of items will experience significant slowdowns when first opening the browser extension.