> And when you say "slow" do you mean slow in receiving Grep output,
> slow in displaying the received output, or slow in moving though the
> *grep* buffer after everything was displayed?

Slow in receiving, slow in displaying, or but not slow in moving
though the hidden parts of long lines.

>> Then instead of hiding long lines using font-lock,
>> I tried to do the same using the process filter:
>> 
>> (defun grep-filter ()
>>   (save-excursion
>>     (let ((end (point-marker)))
>>       (goto-char compilation-filter-start)
>>       (forward-line 0)
>>       (while (< (point) end)
>>         (let ((eol (line-end-position)))
>>           (when (> (- eol (point)) 64)
>>             (put-text-property (+ 64 (point)) (line-end-position)
>>                                'display "[…]"))
>>           (forward-line 1))))))
>> 
>> Still very slow.
>
> Same question as above.

Slow in receiving and slow in displaying.

>> Then tried to delete the excessive parts of long lines:
>> 
>> (defun grep-filter-try ()
>>   (save-excursion
>>     (let ((end (point-marker)))
>>       (goto-char compilation-filter-start)
>>       (forward-line 0)
>>       (while (< (point) end)
>>         (let ((eol (line-end-position)))
>>           (when (> (- eol (point)) 64)
>>             (delete-region (min (+ 64 (point)) (point-max)) (line-end-position)))
>>           (forward-line 1))))))
>> 
>> Now Emacs becomes more responsive.  But still output processing
>> takes too much time.
>
> What is "output processing", and how did you measure the time it
> takes?

Measuring visually, it takes too much time to output the long lines.

>> Finally, the last thing to try was the same solution that Richard
>> showed in bug#44941:
>> 
>>   grep -a "$@" | cut -c -200
>> 
>> that gives the best possible result.
>> 
>> I doubt that it would be possible to invent something better.
>> 
>> So the question is should this be customizable for adding
>> `cut -c` automatically to the end of the grep command line,
>> possibly with `stdbuf -oL` suggested by Andreas.
>
> I suggested to request the equivalent of "cut -c" to be a feature
> added to Grep.
>
> Failing that, I don't think Emacs should do something like that,
> especially since 'cut' is not guaranteed to be available.  Users who
> have such problems can, of course, modify the Grep command to do that.

Finally I solved the long-standing problem by customizing
grep-find-template to

  "find <D> <X> -type f <F> -print0 | sort -z | xargs -0 -e grep <C> --color=always -inH -e <R> | cut -c -200"

I'm not sure if something like this could be added to grep, but
here is an example how such a new option could look: