Version: 1.16.3
Forge Version: 34.1.7, tested 34.1.0 and 34.1.1
Mods Installed:

• Inventory Tweaks 1.16.2-1.0.0
• JEI 1.16.3-7.3.2.36
• JMT-MCMT 0.17.54-1.16.3
• OptiFine 1.16.3 HD U G3

Problem: Generating/loading worlds will stop at 100% permanently if OptiFine is installed. Game needs killed.

Logs:
2020-10-05-4.log.gz

Notes:

• I've noticed that in the logs, it says "Preparing spawn area: 0%" but the screenshot shows 100%.
• The mod works fine without OptiFine installed.

Have you tried installing OptiForge alongside Optifine? That fixed it for me.

If I remember correctly, you need OptiForge 0.3.2 for OptiFine G3 or OptiForge 0.3.3 for OptiFine G4 pre1, assuming MC 1.16.3 of course.
https://www.curseforge.com/minecraft/mc-mods/optiforge/files/all

I have confirmed this in 1.16.3 in trying to get Minecraft to work on my old dying laptop

I'll look into this, but it's possible this is going to be a wontfix/cantfix because optifine is a wierd beast that like this mod hacks around with core game internals

As a side note, you CAN still use this mod and optifine if you run a dedicated server with the mod and a client without, but that's a LOT of effort to make this work