The result is that the rallies aren't actually that much longer (check the stats, they aren't).
At Wimbledon they are, a lot longer.
In 2016/17 I took data from this site
http://www.tennisabstract.com/charting/meta.html
and calculated the percentage distribution of points in slam finals since 1978 (AO,FO,W,USO). I plotted the resulting graphs and smoothed the lines to get trends. I posted them here (GPPD), but the hosting site doesn't show them any more and I can't be bothered posting them again. However anyone can go on the site and do the calculations.
I can however show the results for RG and W, and they clearly show what has been happening. Here are the % shots for 4 different rally lengths in 1980/2000/2017/2019*
1-3 shot rally
W (1980) 72% (2000) 80% (2017) 62% (2019) 58%
RG (1980) 45% (2000) 43% (2017) 44% (2019) 39%
4-6 shot rally
W (1980) 24% (2000) 17% (2017) 27% (2019) 22%
RG (1980) 22% (2000) 27% (2017) 26% (2019) 30%
7-9 shot rally
W (1980) 3% (2000) 2% (2017) 6% (2019) 9%
RG (1980) 13% (2000) 14% (2017) 16% (2019) 11%
10+ shot rally
W (1980) 1% (2000) 1% (2017) 6% (2019)11%
RG (1980) 20% (2000) 16% (2017) 14% (2019) 20%
So the % of short rallies at Wimbledon has significantly declined, while long rallies (7-9, 10+) has dramatically increased. RG has not changed much. The results for USO and AO are similar to W, but not as extreme because they did not start from such an extreme position. I am not showing them here to avoid cluttering the key points.
When you look at the data the key change point is 2000, which is when W decided the audiences did not want 80% of rallies to be 1-3 points and the next year they changed the surface. In fact all four slams have moved over the years to cluster much more closely in the distribution of rally lengths. They are not the same, but they are much more similar than they used to be, certainly in the 70s. The numbers bear out the argument that surfaces are more homogeneous than they used to be.
* the numbers for 1980/200/2017 are the averages from trend lines through the data. The numbers for 2019 are actuals for Nadal-Thiem and Federer-Djokovic.