Changes made in the microcode for the 3 Feb 1992 release have made a substantial improvement in how quickly the large-format CCDs can be read. It now takes about half as much time to read each pixel, the preclear at the start of the exposure is ten times faster than it was. Changes to the A/D card and the microcodes in August 1996 have sped this up even more; see Q.
Some users are scheduled with a large-format device but wish to use
only a small section of the chip. The way these devices work, you will
get the maximum gain in speed if the region to pick is at low row numbers.
(The column location is irrelevant.) If you wanted a 
section of one of the 2048 chips (t2ka, t2kb, s2ka), you may want to
choose a region at the bottom of the chip: firstcol=768 lastcol=1279
firstrow=1 lastrow=512 in detpars would be a region
at the bottom center of the chip. Using the large chip in this fashion should
add little or no overhead compared to using the equivalent 
device, such as ``t5ha".
Remember too that the read time is dependent upon the gain value you have chosen (Sec. 3.1.1). The default value (lowest number of electrons per ADU) yields the lowest possible read-noise, but will take substantially longer to read the chip.