Performance regression in CVS HEAD's *compilation* buffer

[sand@blarg.net]

I updated my CVS tree to HEAD today (2009-02-03, around noon PST),
compiled it, and started using it.  My previous sync had been around
2008-12-01.  During that interval, the performance of inserting long
lines into the *compilation* buffer has dropped by an order of
magnitude.

Here's a simple test case to reproduce:

* In *scratch*, create 96 lines of "x" characters, 64 characters wide.

  <f3> C-u 64 C-u x C-m <f4>
  C-u 95 <f4>

* Put them all onto a single line.

  M-<
  C-u 10000 C-x f
  M-q

* Put an "echo" on the front to make it an invokable command, and copy
  it to a register.

  e c h o <SPC>
  C-x h
  C-x r x c

* Run `compile' with that command:

  M-x compile
  C-a C-k
  C-x r i c
  <RET>

It used to take on the order of 10 seconds to complete.  For example,
on my home machine:

  -*- mode: compilation; default-directory: "~/" -*-
  Compilation started at Tue Feb  3 19:28:40

  echo [...]

  [...]

  Compilation finished at Tue Feb  3 19:28:50

(Output elided.)  With CVS HEAD, this takes well over a minute.  In
both cases, Emacs is completely unresponsive during this display
period.  This is a big problem when running Make with extremely long
(~6kB) output lines.

My home machine is a Debian box running a snapshot of CVS HEAD from
2009-01-11, and I'm unable to reproduce it here, so the performance
drop seems to happened sometime between the 11th and today.

Thanks,

Derek