Why some data such as picture / videos take more time to copy?

All kinds of data are stored internally as binary digits(bits).Procedure for a copying a file is that the computer first copies the file content as bits from the main memory to its RAM buffer and then to the new location in the main memory. In this procedure there is a criteria that only certain amount of bits can be transferred from main memory to RAM at a time which is mentioned by the block size. Ex: 16-bit, 32-bit, 64-bit computers.

Data files are characters or numbers and thus each require just one byte (8 bits) or two bytes (16 bits) respectively. Whereas image files are stored pixel by pixel (smallest unit in the screen) and thus require more number of bits to store them. Thus during copying as more number of bits must be transferred between the main memory and RAM, the time taken for copying pictures is more than simple data files. Also the time taken for copying the same picture varies from computer to computer depending on the block size.