List Count

[Blind Anagram]

I would be grateful for any advice people can offer on the fastest way
to count items in a sub-sequence of a large list.

I have a list of boolean values that can contain many hundreds of
millions of elements for which I want to count the number of True values
in a sub-sequence, one from the start up to some value (say hi).

I am currently using:

   sieve[:hi].count(True)

but I believe this may be costly because it copies a possibly large part
of the sieve.

Ideally I would like to be able to use:

   sieve.count(True, hi)

where 'hi' sets the end of the count but this function is, sadly, not
available for lists.

The use of a bytearray with a memoryview object instead of a list solves
this particular problem but it is not a solution for me as it creates
more problems than it solves in other aspects of the program.

Can I assume that one possible solution would be to sub-class list and
create a C based extension to provide list.count(value, limit)?

Are there any other solutions that will avoid copying a large part of
the list?