How do I throttle a command in a terminal window?

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http://askubuntu.com – I needed to run convert with a lot of images at the same time. The command took quite a while but this doesn't bother me. The issue is that this command rendered my computer unusable while the command was running (for about 15 minutes). So is it possible to throttle the command by limiting resources (processor and memory) to the command, directly from the command line? This can only work if I add something to the same line before pressing Enter because once I start the process the computer slows so much that it is impossible for example to switch to "System monitor" and reduce priority. (HowTos)