LibJF injects hooks into the resource loader, meaning there is additional overhead for every resource access.
In practice, this results in a five-percent increase in resource reloading time over vanilla, and an eight percent increase in game load times over vanilla (.2 seconds).

Respackopts uses those hooks and additional mixins to work, adding additional overhead.
In practice, this results in a six-percent increase in resource reloading time over vanilla, and an eighteen percent increase in game load times over vanilla (.43 seconds).

The most significant increase is measurable during pack scanning, which takes nearly 100 times longer when comparing Minecraft with Respackopts to vanilla.
This is because Respackopts performs and caches most parsing and computation during this stage.
As the statistic for game loading shows, this is not necessarily noticeable during startup, because the total is still comparably small.
It is only noticeable in the resource pack screen, where scanning is performed every 20 ticks and whenever the screen is resized, which can lead to minor freezes while it is visible.
For this reason, I have created a separate mod that tweaks that screen for increased performance.