The only time I ever had problems with Plex using my g620 chip was trying stream 1080p content to my phone online. Streaming content via my network on my devices (Nexus 7, laptop, ps3, Samsung sII) all worked fine. I wanted to be able to stream to my devices online via internet and the g620 struggled so I bumped it up to a i5 2400 for the HT and it helped, but I found I don't use it as much now (haven't been able to really afford going on vacations as much as we used to) so now the i5 is just sitting on my desk.
This is exactly what I see with my G630 CPU. Good on network and poor to my iphone over internet.