The image sensor includes a first group and a second group of column readout circuits for reading out pixel signals from said pixels. The total number of column readout circuits in each group is substantially less than the number of columns in the image sensor pixel array. Further included is a multiplexer bus system having selection switches for selectively switching pixel signals from a block of pixels in a column as input into the first group of column readout circuits. The multiplexer bus system also selectively switches pixel signals from another block of pixels in a column as input into a second group of column readout circuits. However, when the first group of column readout circuits is reading and storing said pixel signals, the second group of column readout circuits is transferring out the processed signals. Thus, the first and second groups work alternately.