When drip irrigation connectors leak, it is usually because the hole that was punched into the 1/2" hose is too large. This can happen when you try to re-punch the hole because the 1st punch did not go all the through and you can't get your connector into the line. When you try to re-punch a hole it's extremely hard to get the punch in exactly the same spot. Since you are off a tiny bit you actually end up with 2 holes, the one you put the connector into and right next to it a tiny hole that sprays out extra water. I've tried plugging the hole with the large end of the drip irrigation plug and sometimes that will stop the leak. When it does, then I just punch a new hole and try again. When it doesn't, then I use a silicone based glue to put around my connector. I get the type of glue that is good for fish tanks; I figure if it won't kill my fish then it won't contaminate my food.
It's difficult to answer this question, without actually seeing the plumbing leading to each fixture. Reductions in pipe size, differences in pipe material, pipe length, number of elbows/tees, etc. These could all contribute to a reduced flow.
If you had a leak large enough to reduce the flow by half, you'd likely either notice it as water damage, or on your water bill. Reduced flow being the only symptom, a leak would be low on my list of things to check.
I'd likely start with the sillcock itself, to make sure it's operating properly. If the valve isn't opening all the way, or it's gummed up with junk, you'd definitely see a reduction in flow. Turn off the water to the fixture, and remove the sillcock.
NOTE: That's a frost-proof anti-siphon sillcock, so it actually connects to the supply plumbing somewhere inside the house. So you'll want to have a bucket under that joint, to catch any drippings when you remove the sillcock.
If everything checks out there, start working your way back along the supply. Look for pipe size reductions, material changes, stuck valves, or any other potential restrictions.
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TRY a POTATO ! just cut a piece of potato and "screw" the potato into the female end-- it just might grab enough for you to unscrew the broke male threads -- it is safe on your hands and cheap to try - won't damage anything-- I have used this to get out broken light bulb parts in a socket too! GRANDPA taught me this decades ago!