@wauwau100. Thanks for posting your question. Actually, I answered a similar question to this a month ago. the answer I gave there applies to your situation. Please have a look at my post: http://support.michaelgilkes.com/topic/folders-not-collapsing-1#post-808
What you are experiencing is not a problem or an error. The fact that you are displaying over 4000 files (and thanks for stating that clearly at the start of your post, instead of me having to find it out) should hint to you as to why it takes so long for the page to load and for the collapsing to happen.
As my post explains, every browser processes HTML tables in the same way. They MUST load the entire table first before the page can finish loading. So, because your table has over 4000 rows, it will take a very long time to load. This occurs because the browser is not only rendering a web page, but also loading all these thousands of rows into the DOM object memory, and each row has its own properties.
Although I am proud of the fact that my plugin does not limit anyone in terms of how many files it can list, persons need to understand that listing thousands of files does not correlate to a good user experience (whether you are doing this manually, or using my plugin, or any other plugin). The plugin does its job perfectly, but we need to use it effectively. The post link I listed above explains this better.
Hope this helps.