Improve performance of Xee

I've noticed a regression in performance of Xee from the time we've introduced micro benchmarks.

Before:
```
open_dataset():avg=11.89,std=3.99,best=6.71,worst=22.73
open_and_chunk():avg=11.44,std=3.40,best=7.46,worst=20.65
open_and_write():avg=58.49,std=12.15,best=48.75,worst=84.94
```

Today:
```
open_dataset():avg=58.46,std=11.26,best=44.83,worst=76.46
open_and_chunk():avg=51.79,std=8.26,best=39.26,worst=69.36
open_and_write():avg=102.41,std=11.80,best=90.74,worst=129.80
```