Hi Trent, thanks for the answer.
for the last few weeks I have problems with unresponsive konsoles (and gnome-terminals as well, I tried) under KDE with Debian 7.8.
Please not: I meant konsole, the KDE terminal, not the text console of a Debian system. I may have confused you here, sorry.
They seem to be extremely slow (I have to wait for 10 seconds after entering a simple command) and I believe it happens only when I am running commands with large output in other windows (e.g. a ssh session to a FreeBSD server and "make buildworld")
When this happens, check loadavg (it should be below N, where N is the number of CPU cores); check for procs in D state; check iostat; check free.
My problem is not the "make" on the FreeBSD server, it is the graphical terminal(KDE's konsole or gnome-terminal)
In my experience with fbcon, large output (e.g. dmesg or cat /var/log/syslog) will slow the entire system noticably, but switching to another vt (Ctrl+Alt+F2) "fixes" it.
I also noticed it running scren inside xterm, but it was MUCH MUCH MUCH faster to recover -- it's the main reason I start X by default these days.
Until now I have not seen it inside xterm. It seems to be the terminal application itself, konsole or gnome-terminal, which holds all terminals in one application. It means, KDE's konsole becomes unresponsive. I can switch to IceWeazel (e.g.) but the drawing of another konsole window takes a long long time.
Are you running screen / tmux?
When this happens, is the "lots of output" window onscreen? Try minimizing it (konsole) and switching window or detaching (screen), so that fewer layers are trying to render the "lots of output".
I am running things sometimes in screen but I am not sure whether it was always the case when I experienced the problem. Sometimes the "large output" window is a hidden tab or in a different workspace. So the actual rendering does not seem to be the problem. The problem appeared only recently.. so I wonder what kind of change could have triggered it. I also run KDE's konsole on a much less powerful laptop running PC-BSD + Fluxbox. I have not seen the problem there at all. I actually like the KDE but I always seem to get to a point where some things are becoming unstable. So at the end I usually return to other X11 environments which are lite. I am just installing Fluxbox.. I do not want to worry about my own desktop. I want to concentrate on the servers I look after.. Regards Peter