AI momentum trades are sharply reversing, with Goldman Sachs' trading desk calling the concentrated sell-off in the AI and semiconductor sectors the "worst performance since DeepSeek Shock Day." Goldman Sachs' high-beta momentum portfolio, which had accumulated gains of approximately 70% by June, has now given back all its year-to-date gains as of October. The Philadelphia Semiconductor Index (SOX) has fallen approximately 13% relative to the S&P 500 over the past nine trading days, marking its largest relative decline over a comparable time window in over 20 years. The debate over AI investment costs versus returns, sparked by Meta's earnings report, has become the catalyst accelerating this downturn.