The Bug
Using a custom .png font slows the client down very much, for example, the resource pack -
[media]- contains only one file, which modifies the ascii.png.
In 1.13-pre5:
[media]In 1.13-pre6:
[media]
As you can see, the FPS drops by over 300fps.
Please check if that's still an issue for you in the latest version of Minecraft.