Rails console running incredibly slowly when editing text

In one of my rails apps, the console has started running really slowly when i paste in text, type and (especially) delete text. I can see in top that irb is using lots of cpu. But i don't know how to diagnose this problem any further. It just started happening a couple of weeks ago. I'm wondering if it's possibly readline/wirble related? I use both of those.

I just tried it in another app, pasting in a block of text, and it seems just as bad - the text is appearing at the rate of one char a second! Maybe my command line history has filled up or something? How can i delete it? (for the rails console, not my bash command line history)

grateful for any advice - max

Edit - sorry, should have supplied some system details. Here you go:

System - Ubuntu 10.04
Ruby version - ruby 1.8.6 (2007-09-24 patchlevel 111) [i486-linux]

I just tried plain irb and i have the same problem. It might even be slower, it's pretty much ground to a halt halfway through the block of text i pasted in to test it.

I've rebooted many times (my laptop battery is knackered so i have to restart every time i unplug it anyway).

I'm not in a vm.

I have recently started using RVM (ruby version manager) and it seems to have coincided with that, though it might just be a coincidence. The problematic consoles are happening using system ruby, though, not an rvm.

Here's the output from ps aux | grep irb:

max      12583  0.0  0.0   1756   484 pts/7    S+   Apr11   0:00 sh -c irb  -r irb/completion -r "/home/max/work/rails_apps/millionaire_container/millionaire/config/environment" -r console_app -r console_with_helpers --simple-prompt
max      12584 15.9  2.7  61872 56956 pts/7    S+   Apr11 158:26 irb                                                                                                                                                                                     
max      13981 64.4  0.9  20080 18708 pts/9    R+   09:40  29:51 irb                          
max      14625 21.8  0.6  15020 12628 pts/12   Rl+  10:25   0:20 irb                                             
max      14757  0.0  0.0   3048   804 pts/13   R+   10:27   0:00 grep --color=auto irb