Random reads/writes to the drive will kill the sequential transfer rate that ImgBurn relies upon when burning.
So yeah, make sure the PC isn't doing anything silly like doing a full virus scan, defragging, indexing etc.
Of course when it's not burning, it's a good idea to run a full defrag on the drive - oh and don't use the standard windows one, it's awful. If you need a freebie, use defraggler.