web2py or grok (zope) on a big portal,
First, don't assume that a data abstraction layer will have unacceptable performance, until you actually see it in practice.  It is pretty easy to switch to RAW sql if and when you run into a problem.
Second, most users who worry about there server technology handling a million users never finish their applications.  Pick whatever technology you think will enable you to build the best application in the shortest time.  Any technology can be scaled, at the very least, through clustering.
I am planning to make some big project (1 000 000 users, approximately 500 request pre second - in hot time).
For performance I'm going to use no relational dbms (each request could cost lot of instructions in relational dbms like mysql) - so i can't use DAL.
My question is:

how web2py is working with a big traffic, is it work concurrently? I'm consider to use web2py or Gork - Zope,
How is working zodb(Z Object Database) with a lot of data? Is there some comparison with object-relational postgresql?

Could you advice me please.