Improve your triathlon cycling splits by working on proper form, race specific training, and cycling positioning.
Improving pedaling form is the easiest way to improve cycling speed. I say this because it doesn't require any extra fitness. For example, many cyclists pedal much harder with one leg than the other, or apply pressure unevenly throughout the stroke. These problems can be solved with a few simple pedaling drills. This will equal more speed without any extra effort.
The next step to a faster time is to train very specifically for the event you plan to enter. An athlete planning on racing in a short criterium style road race should train much differently than an athlete planning on doing the 180km of an Ironman. In general, the shorter the race, the higher the intensity. Training intensity should match race intensity. The type of course you plan to race on should also be considered in training. Take the time to see if the course is flat, hilly, hot, cold, windy, etc., and try to match your training grounds to where you will race.
Finally, bike position can play a large roll in your cycling speed. The idea is to find the position that will maximize areodynamics while still maintaining optimal power output and comfort. Similar to training, position should be matched to the event you plan to enter. For example, if you are doing a short prologue time trial, your position may be much lower in the handlebar to reduce drag, while your position for a century race will be higher to increase comfort over the long haul.