Is mongodb aggregation supposed to be used with performance critical Http requests I have been trying to figure out the purpose of the mongodb aggregation framework. What I mean is, I want to figure out whether it is pointed to be used for database administrators for getting meaning out of big data sets, or is it pointed for use within application code for real time queries ?
Is it suitable to use aggregate with a big database to reply performance critical http requests ?

In short: yes.   But keep basic i/o dynamics and physics in mind.
Broadly think of the aggregation framework in the same way you would traditional GROUP BY.
It can be used to gain analytical insight into data sets, both by admins and users / applications.  Depending on the amount of data being analyzed, then the complexity/size of the data, the complexity of the query, and the hardware (machine and storage) will determine the performance profile.  Some aggregations could take mere 10s of millis; others longer.   The more you can filter out material to be analyzed in an index-optimized way using the $match operator, the faster the rest of the aggregation will run.