>It's already had plenty of time to compound over generations, to whatever extent it does.

I'm not sure what you mean, most westerners don't make a habit of marrying their cousins.

Because of this compounding it means that the risks would be much higher than 3-4%, which is what we see in the data.

>I mean, are you really suggesting that a slightly higher incidence of e.g. cystic fibrosis in Pakistani communities somehow makes it more difficult for them to integrate?

It seems as though birth defects aren't the only way that inbreeding negatively affects offspring. It also appears to lower intelligence[0], while also increasing mental health issues.

[0] - https://link.springer.com/article/10.1007%2FBF01068128