Version: (using KDE 4.2.0)
Installed from: Unlisted Binary Package
I'm using the Debian experimental packages.
1) Log in to a remote machine (ssh -CX example.org) (might need to be via a limited-bandwidth connection; 5 Mb/s in my case).
2) Start kmail.
3) Create a new message.
4) Start typing. You'll see each letter added at about 1 per second.
5) For comparison, start kwrite or kate and start typing. You'll see bunches of letters added with lag, but they tend to keep up.
It seems there's an awful lot of X traffic for each letter, one after the other, using the kmail composer.
I guess this is somehow related to bug 167583.
It is slow even with a local X server.
*** Bug 191702 has been marked as a duplicate of this bug. ***
The problem is the composer being slow - so it cannot be fast through a remote session
*** This bug has been marked as a duplicate of bug 119281 ***