Useful transforms require larger blocks and extensive computation. Amplitudes give 2-D spatial freq. Quantizing parameters allocate bits as long as not easily visible. Statistical Coding Huffman codes use fewer bits for common strings. Computation is required for statistical analysis yielding codes.
Space is needed for code tables, in bitstream if done adaptively.
To achieve that need interframe coding, such as motion compensation. 6.3.7a Motion Compensation Divide image into blocks. Determine motion vector for each block that has moved. Baseline sequential is all that is supported by many implementations. JPEG - Architectures Lossless done as predictor and statistical encoder.