Будет ли использование неравных объемов оперативной памяти в каждом слоте снижать производительность?

This page describes the RAM specifications for many of the MacBook laptops. In the section for my laptop (13″ Pro, Late 2011), it says:

Additional Notes: For best performance, fill both memory slots, installing an equal memory module in each slot.

This is not the first time that I have heard of this phenomenon, and it is definitely not specific to Apple. Why is this the case? Other than the obvious decrease in available memory, why would it be worse to run with 6 GB (1*4 GB plus 1*2 GB) than with 8 GB (2*4 GB)?

With equal amounts of memory in both slots, memory can be interleaved so that successive chunks of memory alternate slots. That way, memory access gets distributed to both slots almost perfectly evenly, allowing their bandwidth to combine.

With uneven amounts, memory cannot be interleaved and has to be mapped first to one stick and then the other. A program that is accessing a large contiguous chunk of memory will find almost all of its access going to one stick and there will be no bandwidth combination.

Additional Notes: For best performance, fill both memory slots, installing an equal memory module in each slot.

The likely technical explanation here is that your MacBook (motherboard) supports dual-channel architecture and thus requires two identical memory modules. Benchmarks suggests that it gives a 5-10 percent performance boost.

Why is this the case? Other than the obvious decrease in available memory, why would it be worse to run with 6 GB (1*4 GB plus 1*2 GB) than with 8 GB (2*4 GB)?

When two non-identical memory modules are used, the motherboard will run the memory modules at the speed of slowest module.