Any time MBM is in my modpack on 1.18.1 More Blocks Mod-0.1.9-a -MCV 1.18.1 it takes extremely long to startup. Im talking like a few minutes even sometimes. Can be replicated with just Fabric API + MBM latest versions on 1.18.1
This behavior doesn't happen on 1.18.2 as it starts up at normal speed

Minecraft 1.18.1
Fabric 0.13.3
moreblocksmod-0.1.9
fabric-api-0.46.6+1.18

I've been testing to see why this happens, and it seems to be when there are many potential blockstates it takes a while to figure what models are actually being used. In the update (0.2) that I'm working on, I was testing connected thin logs and it was almost 10 minutes before load. I fixed the issue and now it's back down to 2.5 minutes. It's an interesting quirk.

That being said, the updates for mbm with 1.18.1 and 1.8.2 came out almost at the same time, and 1.18.1 was a quick output with minimal checks on performance because it was the later version.