I came upon this discussion:
- http://stackoverflow.com/questions/6337893/why-is-array-key-exists-1000x-slower-than-isset-on-referenced-arrays

Did the test on my virtual machine running vagrant box with php 5.3.10:
```
array_key_exists( $my_array ) 0.0097150802612305
isset( $my_array ) 0.0034310817718506
isset_( $my_array ) 0.0059220790863037
array_key_exists( $my_array_ref ) 6.4938058853149
isset( $my_array_ref ) 0.0028769969940186
isset_( $my_array_ref ) 6.9043681621552
```

I guess array_key_exists() makes a copy of the hashmap prior testing and this slows it down.