Map / Reduce Impacts

what impact does running views (map / reduce ) multiple times on the system ? for example, if i have a trade data store and if the clients are always requesting for trades in a particular portfolio (where portfolio name is an attribute in the document - each query with different names) which is getting updates at frequent intervals?

the query would be something like select * from tradestore where portfolio name in ('name 1' , 'name 2')

and iam expecting to have more than 2M active documents in the store at any given point in time.