bug#36566: 27.0.50; debug is sometimes horribly slow

[Drew Adams]

> sometimes it takes 10 or 20 seconds until the debugger pops up.  I
> debugged code using widgets (who are complicated long lists).  But both
> the number of frames and the length of the printed lists were not
> exorbitant - a quite normal situation.
> 
> When I revert
> 
> e09120d68694272ea5efbe13b16936b4382389d8
> Add backtrace-mode and use it in the debugger, ERT and Edebug
> 
> the debugger becomes fast again and pops up in a reasonable time.  With
> the commit, the debugger is quite unusable in some cases: stepping
> (`debugger-step-through') takes ten seconds per hit etc.  Not good.
> 
> FWIW, I think I very much appreciate the cited commit - I didn't yet
> make use of the added features but it seems to be cool.  I hope we can
> find a solution here.  At least there should be a way to get a faster
> behavior.  BTW, I used the profiler and found that most of the time is
> spent while garbage collecting.  My first thought was that cl-print may
> be the culprit but that doesn't seem to be the case.

BTW:
Is there a way to "turn off" the feature introduced
by the commit, even if most people want it on most
of the time?  (I'm not familiar with the feature,
but it sounds like something some users might want
to turn off sometimes.)