How do I improve video perfomance on my dual core Macbook Pro?
Assuming that you are not using the full computational power of your CPU (if you were, then that would be the end of the optimization process, overcloking put aside), the only thing you can optimize is cooling.

CPU can get throttled quiet heavily at high load after a certain point if the cooling system fails to evacuate the remaining heat, for two reasons : 


Fans and radiator can get quiet dirty, hence obstructing the airflow
The heat paste can be aging and/or of poor quality, not transferring heat between the CPU (the place were heat is generated) and the heatsink (the place were it is evacuated by the airflow), hence the heat buildup and necessity to throttle the CPU to prevent it from damaging itself.


So, as for the solutions : 


Cleaning the fan(s) AND radiator with pressurized air will get you a performance output proportional to their dirtiness and is quiet straightforward (just need a a Phillips and T6 screw driver).
As for the state of your thermal paste, and can diagnose it easily by putting your computer under a maximum load and looking at the reading of temperature sensors (the iStat Nano widget is enough) for both CPUS and Heatsink; too much a temperature difference between your CPU (should be 90-95°C) and your heatsink(should be more than 60-65°C) is the sign that could benefit from changing it. Be careful, as it is a very demanding and precise work, but could give you the extra performance you are looking for (don't expect too much of it though, maybe 10-20% ?).


Guides are available on iFixit for all these operations.