You can prune any time after the tree becomes dormant. Generally the recommendation for most species of fruit trees is in late winter, January or February, but before visible bud development. Weather permitting, you can do so earlier if you prefer :-)

Anytime but...right before new growth appears is best because the cuts/wounds heal over faster when not dried up, [fresh].

Why do you think production has fallen off?

I wish I knew. Several years earlier there was so much fruit, neighbors would come and pick some. The fruit quality has also been affected. I think because of its size it is harder to maintain. That's what makes me think trimmed tree will get more air and sun, and will be easier to maintain. Konrad this is not an ornamental crab. Fruit drops to the ground. I have a small ornamental weeping crab with small red fruit. It looks lovely in the snow with red fruit hanging on the cascading branches. In late winter robins and cardinals come to eat it. Last winter, a robin protected the tree from other birds.

>>Konrad this is not an ornamental crab<<

I didn't say it is..

As for low production...perhaps disease/pest is taking over.

Picture would help

Whacking it back is not likely to help it, could in fact be quite harmful. There may be something completely different you need to do to increase production.

For instance many years ago I went to a fruit day at a local Extension experiment station where they had experts from all over giving talks. One party from New York described during their bit how declining yields in commercial apple orchards there turned out to be due to a nitrogen deficiency. Yes, nitrogen - and not phosphorus or potassium.

The tree looks good in the summer. It is quite possibly a deficiency in some nutrient. The fruit is small in size compared to several years ago. However, I still prefer smaller tree.