StreamConfig::validate() is consuming about 0.5% of index.php wall-clock time, which seems excessive for what it does:
At a glance, I would guess the main contributors are the eager evaluation of var_export() and the regex validation.
We could optimize this by only running var_export() when $settings[self::STREAM_SETTING] is actually unset.
I question the value of this up-front validation entirely, though. Might it not be better to simply let it fail later if the config is bad instead of incurring this cost on every request? In other words, what would be the harm of deleting StreamConfig::validate entirely?