I guess it is not only about pace.
Current Messi SEEMS to be less effective on his dribbles, even the low speed ones.
He SEEMS to lose more balls than in the past, even when playing good.
Maybe the opponents finally learned something about him, after so many years of pure humiliation.
Maybe the age is doing its part.
Or maybe both, I don't know.
But the incredible thing is: he still is the best player of the world, which says a lot about how supernatural old Messi was.
There's not really a huge difference in his dribbling success/failure rate now as opposed to the past.
Per whoscored:
La Liga - per game
14/15 season - 3.8 unsuccessful - 4.7 successful - 8.5 total - 1.24 ratio
13/14 season - 3.3 unsuccessful - 4.6 successful - 7.9 total - 1.39 ratio
12/13 season - 2.3 unsuccessful - 3.8 successful - 6.1 total - 1.65 ratio
11/12 season - 3.7 unsuccessful - 4.8 successful - 8.5 total - 1.30 ratio
10/11 season - 3.9 unsuccessful - 5.6 successful - 9.6 total - 1.43 ratio
09/10 season - 2.7 unsuccessful - 4.1 successful - 6.8 total - 1.52 ratio
UCL - per game
14/15 season - 3.9 unsuccessful - 7.1 successful - 11 total - 1.82 ratio
13/14 season - 3.9 unsuccessful - 3.6 successful - 7.4 total - 0.92 ratio
12/13 season - 2.4 unsuccessful - 1.9 successful - 4.3 total - 0.79 ratio
11/12 season - 3.8 unsuccessful - 4.0 successful - 7.8 total - 1.05 ratio
10/11 season - 3.4 unsuccessful - 6.1 successful - 9.5 total - 1.80 ratio
09/10 season - 4.1 unsuccessful - 5.7 successful - 9.8 total - 1.39 ratio
So you see, he's not really less or more successful now than before in La Liga. His ratio is at his lowest, but is not a huge difference and he is attempting the 2nd more amount of dribbles in his career this year, and from an overall deeper position plenty of times. Also, his ratio is climbing as it was worse at the start of the year. Conceivably, he'll get it on par with other seasons by the end if he can get his foot sorted.
In the UCL, he is destroying people left and right. 11 dribbles per game with 7.1 of those being successful, rate of 1.82 successful per unsuccessful. His dribbling numbers vs City and PSG in 4 games are crazy.
Edited for a mistake in calculation.