rec-mode in Emacs for large files slow (+90k records, 57MB)

[J . P . Ascher]

I’m using the recfiles format for a large database of around 97k records
in a 57MB file.  Browsing records in Emacs with rec-mode is fine for the
first few, but at the end of the file it becomes very slow.

For example, `rec-cmd-goto-previous-rec` takes 16 seconds to go from the
last record to the second to last.  I ran a profile of it and I think
that the issue is that `rec-beginning-of-field-pos` is slow at the end
of the file for some reason.  Viz.

```
     Samples    %   Function  
        1734  99% - command-execute
        1734  99%  - funcall-interactively
        1531  88%   - rec-cmd-goto-previous-rec
         966  55%    - rec-goto-previous-rec
         964  55%     - rec-beginning-of-record
         964  55%      - rec-beginning-of-record-pos
         960  55%         rec-beginning-of-field-pos
         230  13%    - rec-record-type
         230  13%     - rec-current-record-descriptor
         230  13%      - rec-current-record
         228  13%       - rec-beginning-of-record-pos
         227  13%          rec-beginning-of-field-pos
           2   0%       + rec-parse-record
         215  12%    - rec-show-record
         214  12%     - rec-narrow-to-record
         122   7%      + rec-beginning-of-record-pos
          92   5%      + rec-end-of-record-pos
           1   0%     + rec-update-mode-line
         120   6%    + rec-current-record
         203  11%   + counsel-M-x
           4   0% + redisplay_internal (C function)
           0   0% + ...
```

Does anyone have any ideas on speeding this up?  If I disable rec-mode,
Emacs has no problem moving page by page at the end of the file: it’s
not so big that it hits the big file problem yet.

Apologies if this is the wrong forum.  I’m not sure where else to ask.


-- 
J. P. Ascher
  jpascher.org - Edinburgh, UK