testing its performance without write, is pretty tricky and doesnt really provide usefull data.
you can "test" it by reading specific files on your disk multiple times i suppose, eventually theyll be cached due to the constant read and then youll get to see the cache speed but youll also have wasted cache space on file you dont normally use so i wouldnt recommend it.
if you want to get a number for the regular use. then assuming only l1 cache, its pretty close to a improvement equal to the cache hit rate (since when the cache is hit, its read from ram and is in the multiple gb/sec speed).
if you use l2 cache, then it would basically be at worst, the difference between the speed of your l2 drive and your cached drive times the cache hit rate.
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