'Jerking' is a guess but it sounds right to me - search 'jerky mouse cursor' (for example) in Google for lots of hits (see first link below, where the jerky cursor is the result of system load, like in your text). I would leave 'effect' out - it's not especially necessary in the English.
FWIW, 'mouse lag' is the term for a delay in the reaction of the mouse cursor to mouse movements (see second link below for an example).
So I would translate the sentence as follows: "This [system load] causes considerable mouse lag and serious jerking of the mouse cursor when you try to select or move windows and elements."

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Note added at 24 mins (2005-05-09 11:21:24 GMT)
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Just refreshed now and saw arum\'s answer.