I know I've seen something about this topic. I want to say it was from LTT but I can't find the video.

I didn't say anything about it being faster. I said utilize it better. Lower latency can be a big help as it allows quicker access. Think of HDD vs SSD. The biggest advantage in the beginning was the much lower latency SSDs provided. Made things a lot snappier even if the speed/throughput wasn't all that different.

I don't know what kind of difference we're taking about here, or how much real world preformance benefits there are but there's a reason CPUs have caches on the die.

And that doesn't include whatever other benefits shorter traces provide. Less voltage drop might be helpful.

But, flexibility must still be better than those gains else most manufacturers would have switched. At some point you start running out of better ways to improve performance though. That's why things are going back to being integrated with the CPU again.