Provide a Debug ID for a slow tag operation. Also, separate from that, try in different collections and the library root, and see if it's slow in all cases. (For example, it's more likely to be slow within a saved search.)

I just am curious--why are tags the one element of Zotero that takes an uncomfortably long time to operate?

As a general rule, don't assume problems you face are universal. Tags certainly don't take 12-14 seconds to delete for everyone, or there would be hundreds of reports. (There are a handful.)

mareike: How recent is this computer? We'll do some tests with many tags, but that does look slower than it should be.

Is there any chance to get a tag-editor in which it will be possible to delete several tags at once?

There are no immediate plans for this (though someone could write a plugin for it), but we'll keep it in mind for future development.

In another thread I found instructions how to delete ALL tags with the help of a Firefox SQLite Plug in. But for some reason, the tags are only delete here, not if I open Zotero on other PCs, so I guess the tag-removal is not synced with my account?

Correct. Direct SQL manipulations (unless you know exactly what you're doing) don't work with syncing. Changes need to be done through Zotero's JavaScript API (or the server API, but that doesn't yet support deleting tags).

Something else: I disabled disable “Automatically tag items with keywords and subject headings”, but still I get automatic tags when downloading data.

You won't with all items, but I believe this happens for translators that hand off to the more generic translators such as RIS. Simon would have to comment on the feasibility of honoring that setting in such cases.

So, maybe it's helpful for someone. Here's how I removed all tags not only locally, but also on zotero's server (I am working with the Firefox plugin):
I followed this instruction concerning SQL manipulation, given by "lmullen":
http://forums.zotero.org/discussion/4051/remove-all-tags/

1. Back up your library (best as Zotero RDF keeping notes and files)

2. Download SQLite Manager for Firefox

3. Search for the Zotero database. You find its path under Zotero Preferences -> Advanced -> Show data directory. The name of the file is "zotero.sqlite"

4. Create a copy of this file (name it for example zotero-copy.sqlite)

5. Open zotero-copy.sqlite in Mozillas SQLite Manager

6. Execute the following two commands one after the other:

delete from itemTags

delete from tags

7. Rename the original library (for example to zotero-original.sqlite; I kept it as a back-up)

8. Rename your manipulated copy (zotero-copy.sqlite) to zotero.sqlite

9. Open zotero and see if all tags are gone

10. Export your library (again in Zotero RDF Format, keeping your files and notes if you don't want to loose them)

11. Delete your complete library from the zotero server (I also had to remove manually all files from my webdav server)

12. Import your just exported library


IMPORTANT:
I don't give any guarantee for anything, but for me this worked fine.
And don't do it unless you're sure you want to loose ALL tags.
If you just want to remove automatic tags, look into the link above. There are some instructions on how to do SQL manipulations for this purpose.

Well, I can not deny that I would have preferred an easier way to edit the tags and keep all other information ;-)