A GPU that is commercially available, is a PCB in mass production hence efficient in production from a cost standpoint. FPGA development boards are also available. But high-end boards will be much more expensive than an off-the-shelf GPU card for a desktop PC for example. FPGA also needs the implementation written in any HDL and it needs to be verified. Hence you will spend time on the custom FPGA implementation. Hence a GPU is the fastest and cheapest solution but not the most efficient.
Hardware and software advisor for tech startups. ASIC, FPGA, RPi, Arduino, AI, robots, drones, blockchain, Machine learning, vision processing, IoT and 3D printers are my fields of expertise.