St. Petersburg's restaurants and cafes - independently on the level and price - are united by one characteristic: to eat quicker than within 1.5 hours is practically impossible. Service is very slow. Everywhere. One needs to consider it building up plans.
The only exclusion from this rule is fast food. There are some Russian style chains which are very good. But you will hardly find such place next to one of the famous sites. So count your time making a program carefully.