Interesting, the story changes when switched to AnyRef/String:

```scala
scala> th.pbenchOff() { val b = mutable.ArrayBuffer[AnyRef](); (0 to 10000).foreach { b += _.toString } } { val b = mutable.ListBuffer[AnyRef](); (0 to 10000).foreach { b += _.toString } }
Benchmark comparison (in 17.02 s)
Significantly different (p ~= 0.0026)
  Time ratio:    1.00751   95% CI 1.00281 - 1.01220   (n=20)
    First     339.6 us   95% CI 338.4 us - 340.7 us
    Second    342.1 us   95% CI 341.0 us - 343.2 us
```

```scala
scala> val ab = mutable.ArrayBuffer[AnyRef](); (0 to 10000).foreach { ab += _.toString }
scala> val lb = mutable.ListBuffer[AnyRef](); (0 to 10000).foreach { lb += _.toString }
scala> th.pbenchOff() { var b = ab; while (b.nonEmpty) b = b.drop(5) } { var b = lb; while (b.nonEmpty) b = b.drop(5) }
Benchmark comparison (in 37.56 s)
Significantly different (p ~= 0)
  Time ratio:    2.77955   95% CI 2.73712 - 2.82198   (n=20)
    First     57.23 ms   95% CI 56.41 ms - 58.05 ms
    Second    159.1 ms   95% CI 158.2 ms - 159.9 ms
  Individual benchmarks not fully consistent with head-to-head (p ~= 0.0050)
    First     60.32 ms   95% CI 57.83 ms - 62.80 ms
    Second    154.6 ms   95% CI 152.8 ms - 156.4 ms
```