Performance issue in project.el

[Ergus]

Hi:

Using tramp I tried to use project.el with a command like
project-switch-to-buffer and it took like 10 minutes to complete.

I ran a profiler and I found that most of the time was taken by:

project-current called in a loop for all the opened buffers it calls
project--find-in-directory that calls
project-find-functions and there is going all the time.

I had global-tags-try-project-root (a function from an external package
tat call gnu global program... something like gtags). And I optimized
that function to use a cache and avoid calling the program for every
buffer creating a cache for it.

Something like:

```
(defvar global-tags--dbpath-cache (make-hash-table))

(cl-defun global-tags--get-dbpath (&optional (dir default-directory))
  "Filepath for database from DIR."
  (pcase (gethash dir global-tags--dbpath-cache)
    ('empty nil)
    ('nil
     (puthash dir 'empty global-tags--dbpath-cache)
     (when-let*
       === BLA ===
       (when (file-exists-p dbpath)
         (puthash dir dbpath global-tags--dbpath-cache))))
    (code code)))
````

Now the time is half than before but still very slow to use the command
(around 3-5 minutes to complete) and running again the profiler I get this:

        5637  89% - command-execute
        5549  88%  - byte-code
        5549  88%   - project--read-project-buffer
        5549  88%    - let*
        5336  85%     - read-buffer
        5323  84%      - ivy-completing-read
        5323  84%       - ivy-read
        4941  78%        - ivy--reset-state
        4941  78%         - ivy--buffer-list
        4941  78%          - internal-complete-buffer
        4941  78%           - #<lambda -0x1a357caf01243d61>
        4941  78%            - and
        4941  78%             - equal
        4941  78%              - save-current-buffer
        4941  78%               - project-current
        4941  78%                - project--find-in-directory
        4548  72%                 - project-try-vc
        4537  72%                  - vc-responsible-backend
        4478  71%                   - #<compiled 0xd3f2e32af0966f7>
        4478  71%                    - vc-call-backend
        4478  71%                     - apply
        1470  23%                      + vc-svn-responsible-p
        1142  18%                      + vc-bzr-responsible-p
         970  15%                      + vc-hg-responsible-p
         390   6%                      + vc-git-responsible-p
         156   2%                      + vc-cvs-responsible-p
         126   2%                      + vc-rcs-responsible-p
         108   1%                      + vc-sccs-responsible-p
          98   1%                      + vc-src-responsible-p
          57   0%                   + tramp-file-name-handler
          11   0%                  + vc-file-getprop
         393   6%                 + global-tags-try-project-root
         375   5%        + read-from-minibuffer
          13   0%      + if
         213   3%     + project-current
          88   1%  + funcall-interactively
         572   9% + ...
          51   0% + timer-event-handler
           8   0% + redisplay_internal (C function)


As you can see most of the time is still taken by project-current and I
can't really understand why:

1) Are so many samples 4548 seems a very high number for only 25 opened
buffers.

2) why project-try-vc still takes so much...? Specially for unfrequent
vc systems in our days like svn or bzr that I am not using.

3) If vc-handled-backends has these value by default: '(RCS CVS SVN SCCS
SRC Bzr Git Hg Mtn) why is there some time taken by vc-hg-responsible-p
that is latter than git in the list?

In any case:

Maybe (I think I mentioned this before) `project.el` needs a sort of
cache to speedup some functions like `project-current` that are called
very frequently.

Best,
Ergus