Will microwaving lemons make them easier or more efficient to juice?

It makes the lemon easier to squeeze. I think it has the most effect on the peel; it's softer and more flexible when warm, so you're able to get more juice out of it than you could otherwise if you're juicing by hand. That's especially true if you're trying to juice several lemons - you'll just get tired and stop being as thorough if it's harder.

It definitely still makes it easiser if you have the squeezer-style juicer, since those do basically turn the fruit inside out in the process. It probably doesn't help as much with the kind that you push the fruit down onto.

In my 900W microwave it takes 1/2 inch water 1 minute to boil and around 30 seconds to become hand-hot, (I know it may take a bit longer when heating a lemon with the skin acting like an insulator although this effect will be lessened due to the high oil content of the zest). You probably won't 'boil' the lemon however if you did it would produce by far the most juice as the water in the cells would turn into steam and burst creating a semi-juiced lemon before you've even cut it in half.

A cell membrane is made up of many intricate structures, one of which is fatty acid tails. When these melt the cell becomes more 'fluid' and becomes more permeable. This fatty acid melts at around 65 degrees. This would create a lemon that is a lot more flexible and juicy. Cell membrane diagram.

Based on this evidence, I personally would say that microwaving a lemon actually could help with juicing, the effect will be most noticeable when you're juicing a lot of lemons without a juicer.

I cannot think of any physical reason why this should be so, and I don't believe it

Have you tried it? Buy two lemons, nuke one of them, and squeeze them both. Measure the juice that you get from each. Better, have someone else (who isn't aware that the lemons are different in any way) squeeze the lemons and tell you if they thought one was easier to squeeze than the other. It'd also be interesting to warm up a lemon in a low oven or just a bowl of hot water.

My own experience is that a short spin in the microwave can make limes easier to squeeze. Limes are often small and very firm, and getting any juice out of them at all is sometimes difficult. The microwave trick does seem to help with this.

I use to warm lemons (and oranges) letting them for a while (maybe 4-5 minutes) in a pot of warm water. They do release more juice. If you feel skeptical about microwave, you can try this way.

Softening lemon with water and all that jazz takes time. Best is microwave, just 30 - 45 seconds is enough to make it soft to squeeze more juice.