Fastest memory is cache memory.
Registers are temporary memory units that store data and are located in the processor, instead of in RAM, so data can be accessed and stored faster. Cache memory is extremely fast memory that is built into a computer's central processing unit.
Cache memory is a small-sized type of volatile computer memory that provides high-speed data access to a processor and stores frequently used computer programs, applications and data. It is the fastest memory in a computer, and is typically integrated onto the motherboard and directly embedded in the processor or main random access memory