You're not running a multi MB image but a small block thereof. Not that 5-6KiB LUT matters, but the extra conditional multiply ops or power or logarithm functi

You're not running a multi MB image but a small block thereof. Not that 5-6KiB LUT matters, but the extra conditional multiply ops or power or logarithm function evaluations do. Latter are need a few MACs for a good approximation (at least 12 bit precise, preferably 16 bit - like 5th order polynomial), former have masks, worse for SIMD.