“Once everyone moved from CPU calculations to GPU calculations,”
They moved from GPUs to FPGAs to ASICs several years ago :-) ... they’re using racks and racks of ASICs doing the operations to mine BTC now ... ASICs are the lowest power, highest performance solution (and also the reason why it’s not very cost effective to mine BTC on your home PC these days since China all but cornered the market on mining BTC cheaply, quickly, and efficiently).
Just to clarify something :
An ASIC (application specific integrated circuit) is a device that performs a specific task. In the case of bitcoin mining, the ASIC is a circuit dedicated to computing the “stuff” it needs to successfully mine BTC.
This differs from a CPU in that a CPU has “overhead” to compute something. A CPU is a more “general purpose” solution for various problems ... the tradeoff is some performance. A GPU has various task processors that can handle the BTC computations a bit more efficiently (and in parallel), but, again, there is some overhead involved and GPU typically require a lot of power (as in electrical power).
FPGAs are basically configurable digital circuits. An FPGA will eliminate a lot of the CPU/GPU overhead and is pretty close to an ASIC. The price you pay (compared to an ASIC) is overall performance. Since the FPGA is a configurable device, it is a bit slower compared to an ASIC since an ASIC’s dedicated circuitry can be optimized for things like propigation delay, clock skew, etc. In an FPGA, these resources are at fixed positions in the device, so you’re going to get a performance hit compared to the ASIC.
The big tradeoff between ASICs and FPGAs is one of cost ... it costs north of $1M to make an ASIC these days ... FPGAs can be purchased for far, far less. However, it seems like the Chinese can afford the $1M cost involved with fabricating the ASIC since they seem to be using thousands and thousands of these devices on their BTC farms.