try normal approach
this approach depends on RAM.
it is only faster when using smaller raster data.
Do not try this way if you use big raster data.
```{r}
system.time(stk[stk <= 0.5 ] <- NA)
summary(stk)
```
```{r}
system.time(stk[stk <= 0.5 ] <- NA)
summary(stk)
```
user system elapsed
0.984 0.406 1.403