The "roll" benchmarks in the get executable show the difference:

  get roll
  benchmarking roll/foldr
  time                 547.4 ms   (537.8 ms .. 553.5 ms)
                       1.000 R²   (1.000 R² .. 1.000 R²)
  mean                 549.4 ms   (547.2 ms .. 550.4 ms)
  std dev              1.857 ms   (0.0 s .. 1.896 ms)
  variance introduced by outliers: 19% (moderately inflated)

  benchmarking roll/foldl'
  time                 434.7 ms   (426.4 ms .. 443.1 ms)
                       1.000 R²   (1.000 R² .. 1.000 R²)
  mean                 433.7 ms   (432.3 ms .. 434.7 ms)
  std dev              1.472 ms   (0.0 s .. 1.696 ms)
  variance introduced by outliers: 19% (moderately inflated)

The "Integer/decode" benchmark shows the actual decoding speed.
First the old implementation based on foldr:

  get Integer/decode
  benchmarking Integer/decode
  time                 552.9 ms   (540.2 ms .. 569.3 ms)
                       1.000 R²   (1.000 R² .. 1.000 R²)
  mean                 558.2 ms   (556.1 ms .. 559.3 ms)
  std dev              1.824 ms   (0.0 s .. 1.906 ms)
  variance introduced by outliers: 19% (moderately inflated)

The new implementation based on foldl':

  get Integer/decode
  benchmarking Integer/decode
  time                 457.5 ms   (406.2 ms .. 505.6 ms)
                       0.998 R²   (0.994 R² .. 1.000 R²)
  mean                 455.9 ms   (448.3 ms .. 462.3 ms)
  std dev              10.11 ms   (0.0 s .. 11.02 ms)
  variance introduced by outliers: 19% (moderately inflated)