Google Instant will most likely never come to GMail search. The speed at which GMail queries are returned simply aren't fast enough for real-time search, and it's unlikely Google has a monetary incentive to improve the speed to this level. Remember, GMail search is a feature; Google Search is a business.

There are a few reasons GMail search is slow:

* GMail searches must search a genuinely real-time index of your e-mails. An e-mail must be searchable the instant it's created. Web search has an inherent latency which allows Google to do offline processing to improve search speed.

* It follows that, GMail must be an exact match, while Web search is an approximation.

* Popular queries can't be cached and returned quickly.

Source: http://www.quora.com/Why-does-Gmail-take-longer-to-search-my...