The difference between 1990 and today is the amount of mid-inning pitcher changes, as managers are looking for R/L matchups between pitchers and hitters late in games. Which in turn leads to more commercial breaks.
But you can't deny that pitchers and hitters themselves cause more delays than they did 30-35 years ago. Hitters stepping out of the box between pitches to adjust their gloves, apply more pine tar to their bats, etc. Pitchers stepping off the rubber to rub up the ball, compose themselves, etc.
Like I mentioned earlier, look at games from the 70's and watch the pace. There are plenty of old games on YouTube. Pitchers stayed on the mound, looked for their signs, and delivered the pitch. Hitters stayed in the box. The game had a much better flow than it does today.
That's indisputable to anybody who watched games in the 70's and early 80's, and who watch games today.