The filter makes Opus calculate the folder size.
If the filter is checking a folder size is 0 then the calculation will stop as soon as something (with non-zero size) is found below the folder, so it won't calculate the whole size if it isn't needed.
From looking at Process Monitor while scrolling through a test folder with 2500 empty sub-folders, it also looks like Opus only calculates size info for the folders IN VIEW. You can also see this is the case by a slight delay as you scroll where the folder icon is initially normal color then very quickly changes to what the label has defined.
I think that approach also greatly mitigates the potential performance impact concerns. I'm actually going to use this sort of empty folder label filter myself - so thanks for raising up the questions 
.
@Leo: while looking at ProcMon I noticed this:
Date & Time: 1/9/2014 4:03:49 PM
Event Class: File System
Operation: QuerySecurityFile
Result: BUFFER OVERFLOW
Path: P:\test\1 - Copy (52)
TID: 1736
Duration: 0.0000041
Information: Owner, DACL
...should we chat on this in a separate topic if this is something to be concerned about? It happens multiple times for each folder being queried in the current view.
I can't get my empty folder label to work (v11.3). In fact, folder sizes are showing sporadically at best (and typically not at all). A bug?
Folder size calculation normally only happens when you explicitly request it (e.g. via Edit > Calculate Folder Sizes, or by hovering the mouse over a folder if the tooltip is configured to show the folder's size).
It can be set to happen automatically (see the Folder size claculation FAQ) but you wouldn't normally want that as it causes lots of extra disk access for information that is usually not wanted.
As for the label, if you show us how you have configured things we might be able to see what's wrong.
[quote="leo"]Folder size calculation normally only happens when you explicitly request it (e.g. via Edit > Calculate Folder Sizes, or by hovering the mouse over a folder if the tooltip is configured to show the folder's size).
It can be set to happen automatically (see the Folder size claculation FAQ) but you wouldn't normally want that as it causes lots of extra disk access for information that is usually not wanted.[/quote]
Good to know, Leo. Thanks.
I have it configured exactly as shown in blueroly's post above (second post in this thread, first screen shot). Do I need to have the folder size calculation set to automatic in order for this folder label to work?
Oops... never mind. I thought I had it exactly as blueroly did, but I didn't: mine wasn't restricted to folders only. Apparently, that makes the difference: if I only have size matching 0 bytes (without the type restriction), it doesn't work on folders.
Yes, that's correct.
Here's the explanation of why it's that way, from Filter Clause Types in the manual:
A Size clause can also be used to compare the size of folders. Because calculating a folder's size can be time consuming, you must specifically enable this behaviour by also including a Type clause that specifies Folders Only. For example...
Nice one! Empty folders, they can be anoying!.. o)
I have a small "but".. o):
I'd like to set the "empty label"-filter to only apply to really completly empty folders.
I have folders with intentional, 0-byte files and maybe empty subfolders. Right now, such a folder is labeled "empty" als well, but to my eyes, it is not empty (it has folders and files). I tried around with the filter, but could not get it to check if there are subfolders or files. Can that be done somehow ?! Is there a criteria like "number of files/subfolders" missign to make it work?
I was also looking for a filter to mark only empty folders (excluding 0-bytes folder that are not empty).
Yeah, there are times I'd like to be able to differentiate between 0-byte dirs with 0-byte child items vs truly empty folders.
This could be achieved if the label filter supported a test for file count, or alternatively a test for folder size = empty, rather than 0 bytes. Any chance of support for one or other or both of these?
Regards, AB
+1
What are some of the situations where you need this?
I use zero byte text files as comments / place holders. e.g.
ABC Project - due 30th June 2014.txt
PM is Joe Bloggs.txt
Backups are in ABC2014 on Projects server.txt
Regards, AB
Push space or return before saving the placeholder files. Problem solved.
It's not a problem, just a nice to have. As you know only too well, we DOpus users are never satisfied with our lot 
and of course, we have no way of knowing how hard it is to implement our "simple" requests.
Regards, AB
I use filter label to visually mark "empty" folders. No, not correct, I mark "0 byte" folders right now.
It is a nice time saver while browsing folders, as I can skip some of them, because their are "greyed out" for me.
But very often I still need to look into these "0-byte" folders, because I cannot determine if they are really empty (containing nothing - no flagfiles etc.) or still containing 0-byte stuff, which has relevance.
That tells me what you are doing, but I still don't really understand why you are doing it.
Where do all these empty folders come from and why are you looking through them, or trying to avoid looking through them while looking through other folders in the same place? It's not something that ever seems to happen in my day to day use, but a lot of people seem to be doing it over the years so there must be a reason for it, I just haven't grasped it yet.
What's missing from the columns which can give you subfolder/subfile counts and directory sizes when in these situations?
(FWIW, I dislike that filter label. It has some inherent problems, some of which are not really fixable, and whenever I see a new complaint about how it works it just adds to my belief that people should not use it at all.)
Mhh well, I tell you of course! o)
I encounter really empty folders when:
I encounter 0-byte folders or files:
You surely had to deal with some of those as well, don't you? o)
So it would be nice if you could setup a specific labels for these:
To get there, I agree with aussie, you'd need to be able to check for filecount==0 or foldersize==empty (instead of current "0 bytes", to make sure no files are in there).
Thanks for asking! o)