Speed up autocomplete suggestions

I am created autocomplete using html,javascript and webservice.
IN javascript i taken one textbox(for entering text) and div(for auto suggestions)
I am calling the webservice for auto suggestions on onkeyup event of javascipt using timer function

ex:onkeyup="delayFunction(showHint,500);" this is mentioned in textbox
for every 500mseconds the showHint() executes
but problems is when i am decresing time ,the cursor struct on textbox when we are entering text,if we incresing time,the auto suggestion will be slow.
so how to overcome this problem