Having higher pixel densities just reduces ISO performance.
No it doesn't.
Yes it does.
Someday someone needs to explain to me why this myth persists after a decade of things going the other way despite similar basic sensor performance (QE).
Many things affect high ISO performance.... we can not fixate on just one yet many of us do.
Ultimately, it comes down to light...
A higher quantum efficiency of the sensor will lead to more light being converted into electrons, and will give you increased high ISO performance.
Lower read noise/higher SNR will give you a cleaner signal and will result in improved high ISO performance.
BSI sensors have less circuitry blocking the incoming light, which gives you more usable light, and results in increased high ISO performance.
Finer lithography blocks less light, and like above, gives better high ISO performance.
A cooler sensor (temperature cooler, not trendy cooler
) has less thermal noise and gives better high ISO performance.
And yes, pixel size does matter (sort of).... a smaller pixel will collect less light than a larger pixel and that pixel will have lower high ISO performance than a larger pixel.... but you have to remember that a picture is not made up out of one pixel.... it is the results of ALL the pixels that give you the picture. For example, think of two sensors built with the exact same technology and of the exact same size. One is a 10Mpixel sensor and the other is a 40Mpixel sensor. The pixels on the 10Mpixel sensor, being four times larger, will collect 4 times the light as the pixels from the 40Mpixel sensor and the ISO performance will be 2 stops better. However, we can not forget that the 40Mpixel sensor has 4 pixels covering the same area as each of the 10Mpixel pixels and that if you bin those 4 pixels together in post production you end up with very similar performance to the 10Mpixel sensor. Yes, you will loose a slightly greater amount of area to lithography, but you gain a bit from increased sampling density. Practically, the end result will be the same..... So in the end, larger pixels give you better PER PIXEL
ISO performance but with normalizing the resulting images, very little difference in PER PICTURE
And yes, size matters. A FF sensor will capture more light than a crop sensor and for the reasons above, better ISO performance.