Benchmark and Clean Up _mountImage Strings

[sebmarkbage]

We're currently not cleaning up strings from _mountImage after a component gets mounted. That memory could be freed. But the act of freeing the memory could be more expensive than it's worth. Needs proper benchmarks and a workaround.

[sophiebits]

I noticed this a little while ago. I'm not sure why we need to store it on the child instance in the first place -- PR incoming.