A volume correlation analysis of the ES, SPY and Cash S&P 500 is attached. For intraday, there is no reported cash volume.
We can see on the daily time frame, volume correlation of the ES and SPY to Cash was much higher in the past than now. This adds evidence that reported volume for Cash is suspect.
On the intraday (over the last year), we see on the 30 minute, correlation between the ES and SPY was high (.89), while on the 5 minute it was very low (0.25). The latter is not surprising because the average daily SPY volume is about 100 x higher, so differences will be magnified on very short time frames.
All I can tell you is what works for me on the very short intraday time frames. Market Profile scales to any time frame, but is geared more toward showing supply and demand areas, not indicating likely directional movements. It's possible some simple form of volume analysis can still provide an edge, such as OBV, but I suspect it is materially less so than in years past.