A question for all you window pros....

As part of a bathroom refresh, I pulled off our original casing off to replace and noticed that our 20 year old windows only had insulation going approx 1/2 the depth of the window. Out of curiosity, I removed casing on a couple other windows and found the same - some insulation (fiberglass) or none on certain areas. To my recollection spray foam was not a common practice back then. These were Pella Architect Series, and issues with Pella aside I have always felt drafts, had icing and condensation issues + noise. My question, is that could the lack of (or insignificant application) of insulation cause the issues I have experienced? As a test I added additional mineral wool fiberglass I had to one room to fill depth of window jamb and immediately noticed a significant reduction of sound. Thoughts????