This problem came to light recently on my laptop. When I reply to a message, with Quote Text when replying checked then one particular message that is 140k and has quite a long string of replies in it - all quoted, but without any attachments - takes about a minute to open after clicking reply.
Now the laptop is a bit old and only has a 450mhz CPU. I tried the same thing on a desktop machine with a 1.2g cpu. The message still took 24 seconds to open the reply window. Its the same in V3 and V4.
So I then opened up Netscape where I backup my mail to. This same message opened up a reply window instantly. It seems to me that PM is having a problem with this type of long quoted HTML message.