I noticed, that when booted from a live disc, and the RAM is filled up nearly, the live DVD starts being accessed far more often (probably due to exhausted and squished block cache) and switching tty's get delayed.
Commands, even simple commands such as 'echo test' get delayed.
$ echo test _
Only SysRq is immune, also printing the SysRq+Space help, which lists the commands, happens instantly.
What is the technical explaination?