Attached patch, I think it's self-explanatory so the commit message is
reproduced here:

org-element: Raise gc-cons-threshold for org-element-cache-map

This garbage collects many times when calling
org-get-buffer-tags (such as during tag completion) in a large file
with many tags.  Raise gc-cons-threshold as this may affect other
callers in large files.

Note that org-element-parse-buffer also raises gc-cons-threshold as a
precedent.

This has no effect on most callers and increases performance for
extreme cases.