On my systems P8 starts up in about 7 seconds compared to about 180 seconds for PoserPro. I have a large library, so if I turn off the tree functionality of PoserPro it will load in about 20 seconds.
Filling the library of P8 with items depends on the number of open folders. Normally that will take about 5 seconds. PoserPro does not have a Search function, but P8 search takes about 5-20 seconds depending on the category setting and the number of results.

Regarding memory usage: If P8 is loaded with my unreasonably large library, it will take up 233MB of adress space, which is comparable to PoserPro

So P8 is much more efficient as PoserPro in these points