Problem with losing distance
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You aren't hitting the ball shorter because of your clubs. I used to hit my Ping Zing 2s super far (145-150 yd PW), and they are 20+ year old technology. Gradually I got better and more accurate and my distance decreased, but as I said, I was better and more accurate. A 135 yd PW is still a very long PW. You won't hit player's irons any longer than your Callaways. You may like 'em better because you're in the mood for new clubs (I've been there), but you won't hit them any longer.
I have just experienced the same thing. I took lessons in March (primary goal was to improve with Driver) and went out for the first time last week. Accuracy with irons did improve but my distances have shortened a bit. I forgot to put my range finder back in my bag so I didn't have hard numbers, But I'll see what happens on this Friday. I recognize that I need to adjust, as I lost only one ball, came up about 5 yards short of a water carry.
If you improved your swing, and are able to swing at the same speed, your distance should increase due to solid contact. But if the improvement in your swing requires you to swing slower, then yes you will loose distance.
For example, lets say you hit a driver with a smash factor of 1.35 at 100 mph, that's 135 mph ball speed. If you improve that to 1.45, but to do so your swing speed lowers to lets say 90 mph, your ball speed is now 130 mph. So that would be a loss of 5 mph, or about 10 yards of distance off the tee.