Interesting we have different reports. My experience was far more in line with parent's in that enabling ECO mode had <5% multicore performance drop (single core unchanged). ECO mode also lowered temps several degrees. Seemed like a no brainer for my use case.

In fact, chips have gotten so fast these days, I am seriously considering running my desktop on a laptop-class CPU. Should be able to run it near silently on air cooling alone with low power consumption.