Perf tweaks

Here are a couple of performance tweaks to address hot spots that are much more noticeable after #435 is applied.

## master

```
$ time ruby -r active_record -e ""

real	0m0.603s
user	0m0.507s
sys	0m0.078s
$ time ruby -r active_record -e ""

real	0m0.603s
user	0m0.507s
sys	0m0.079s
$ time ruby -r active_record -e ""

real	0m0.602s
user	0m0.510s
sys	0m0.073s
```

## master + #435

```
$ time ruby -r active_record -e ""

real	0m0.426s
user	0m0.336s
sys	0m0.075s
$ time ruby -r active_record -e ""

real	0m0.425s
user	0m0.340s
sys	0m0.069s
$ time ruby -r active_record -e ""

real	0m0.429s
user	0m0.327s
sys	0m0.085s
```

## master + #435 + these patches

```
$ time ruby -r active_record -e ""

real	0m0.378s
user	0m0.284s
sys	0m0.079s
$ time ruby -r active_record -e ""

real	0m0.387s
user	0m0.297s
sys	0m0.075s
$ time ruby -r active_record -e ""

real	0m0.378s
user	0m0.291s
sys	0m0.071s
```