Interesting that his performance was essentially identical to the S&P over the past 30 years (Sortino 0.72 vs 0.69): https://testfol.io/?s=jJ4P0GrZxLi

I wonder how much is due to the market becoming more efficient, vs Berkshire's size / market impact?

Probably a bit of both. Berkshire's size means he was limited to the largest companies which are pretty well analysed.

During that period he did some different things which some of his personal money like buying stock in Dae Han Flour Mills, a Korean flour miller that was like 2 times earnings in 2003 but was probably too small a position to make sense for Berkshire. (https://www.netnethunter.com/warren-buffett-cheap-stock-pick...)

How do you figure? The portfolio looks to have performed roughly the same as the S&P.

BRK total return from inception to 2025: about 5,502,284% (55,000x gain)

S&P500 total return over the same period: about 39,054% (390x gain)

$1 in BRK would be $55k today. $1 in S&P500 would be $390 today. Therefore, following the hypothetical 99% drop, 1% of Berkshire return would be $550, still well above the S&P500's $390.

That is wild.