[sathya]
I read somewhere that each zope thread acquires the global python interpreter lock before processing a request and until it releases it the other zope threads are essentially locked out.
The Python GIL (global interpreter lock) affects all code written in Python: only one thread at a time can interpret Python bytecodes in a given process. That's true in or out of Zope.
Does that mean zope threads are eventually serialized and there are no benefits to be gained from concurrent execution ?
No. The GIL effectively serializes threads doing pure computation in Python, but many pieces of the Python implementation release the GIL around calls to known-threadsafe C libraries. The most important examples of that are I/O, of many kinds. For example, if a Python thread opens a socket, the GIL is released around the C-level code that does the low-level socket open, which allows other Python threads to proceed in parallel (of course the thread that does the C-level socket open waits for the latter to finish; it reacquires the GIL after the socket operation is complete and it's ready to go back to interpreting Python bytecodes). Because Zope does a lot of network I/O, this form of parallelism can be very useful for it.