Unfortunately ajax search cannot be instantenious, as it needs the server to return results. Half a second is actually incredibly fast response time, I'm not sure if it's possible to top that.
Instant search engines does not use server side ajax (except google and some very high-end services), but cache the results on the front-end via scripts, which make the page size much higher and the page loading much slower. It basically runs on the clients browser as a script, instead of the server. I'm working on a possible feature in a future release, but there are many limitations – as clients with slower PCs have to be considered as well.
Ajax technology is a safer choice for now, as all the work is done by the server, instead of the client browsers. I'm hoping however to implement such feature in the near future.Best,
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