How to tell supervisor too many lengthy meetings are hurting your productivity?
I think this is a time to be direct and honest: just say you are spending too much time in meetings and it should be reduced.  Say what you should be doing instead.  A reasonable person should not be offended and should know that meetings can hurt productivity.

I also recommend considering changing the way the meeting works to align it with your needs.  When I am in a meeting, if nobody is taking charge, I will try to be a leader and give the meeting some direction.  Remind participants of the agenda.  Add the things you want to do to the agenda.  But be tactful.