Why is "Though tough, through thorough thought, one thinks." a tongue twister?

Some backstory: By chance whilst visiting Ho Chi Minh city, I tried to create a tongue twister for a Vietnamese girl who spoke perfect English. This one did it.

I hadn’t yet asked ChatGPT or any other LLM until now to break down why a very reproducible task (English essentially must be your native language to pronounce this correctly) is near-impossible to do in rapid succession for ESL speakers.

I used o1-preview. Afaik, this is a new tongue twister.

The "ough" sounds are all different.

It's not, it's just hard to read.

If you are a native English speaker it’s nearly trivial. What i’ve found is even ESL speakers who have basically perfect English are thrown off by this. I don’t know why.

>I don't know why

It's the th sounds. Very hard to teach your tongue how to casually produce them if your native language doesn't have them.