Celerons have a much smaller on-die cache. This means that when a large amount of data needs to be procesed (when rendering) the processor gets smaller chunks of the data to process. Meaning that it wastes cycles swapping data in and out of the cache. Most programs are not adversly affected by this, but anything that needs heavy processor usage is much slower on a celeron than on a pentium. A celeron can handle the processing for rendering, it's just going to be slower than with a pentium.