Grouped operations are slow

```r
d <- data.frame(
  g1 = sample(LETTERS), 40000, TRUE),
  g2 = sample(LETTERS), 40000, TRUE),
  g3 = sample(LETTERS), 40000, TRUE),
  x1 = runif(40000),
  x2 = runif(40000),
  x3 = runif(40000)
)

d %>% group_by(g1, g2, g3) %>% summarise(x1 = mean(x1), x2 = max(x2), x3 = min(x3))
```