Different videos can use different codecs/profiles and bitrates. If your CPU needs to do it all "in software", i.e. the dedicated decoding hardware assist isn't helpful because the codec/profile is too complex for the hardware to perform the decode operations and Flash falls back to the software implementation.
Fortunately, Intel x86 chips from the past few years can easily perform all decode operations for practically any codec in software. That said, they also have QuickSync, which is some dedicated video hardware that can be used for encoding and decoding which will reduce the power use of your system if the decoding software (i.e. Flash) supports it. And QuickSync is starting to catch on, especially since Intel released some open source implementations of code which leverages QuickSync.