What are the frame rates you are achieving (high to low)? You should be able to estimate this by looking at the EXIF time, including sub-seconds.
Are you shooting AF-C Focus Priority? (Custom Setting a1) That would be the most obvious and likely reason to see inconsistent burst speeds.
If you shoot Focus+Release Priority that might also slow it down although in practice I have not seen much difference between that and Release Priority.
>> The specific questions is whether the length of time I hold the focus before pressing the shutter can influence this. I feel that with cricket, if I hold the focus in the batsman for several seconds it adversely effects the burst rate achieved. Could this be possible?
Does not "make sense" nor have I ever seen anything like that.