Any idea how I would be impacted by that in the real world?
I'd try both 3x8G 1333MHz (triple channel) and 4x8G 1066MHz (Dual channel) config.
When running GeekBench, yes, you will see a 25% (or more) performance difference for the RAM test. But overall, it's only about 5% difference for the whole machine's score.
I made a 10G RAM disk in both config, and run the Blackmagic disk speed test. There is about 7% difference (3000MB/s vs 2800MB/s) in both read and write speed.
Cinebench, Photoshop, and handbrake speed test, almost no difference (triple channel 1333MHz config always run a little bit better, but only 1-2% improvement). Also, can't feel any difference for gaming, or other normal use.
Therefore, I will say there may be <3% performance difference in most real world application (unless some software is extremely sensitive to the memory bandwidth), but virtually no difference.
My personal opinion, if you want both best speed and max out the RAM size, 3x16G is your best choice. However, if someone like me have 4 smaller RAM size sticks in hand, I prefer to use all 4 slots. I can't see any reason to sacrifice 25% of RAM size for only 3% performance in some applications. Also, if the machine eventually running out of RAM in the 24G config. I am sure the performance penalty will be more than 3%.
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