I'm seeing this behavior as well on two Windows 8.1 clients. I have a fresh install of 3.4.8 on everything. One client is Intel i5-4200Y @ 1.40 GHz and is at about 80% utilization after a channel change. I'm also using a HDHR Prime with CableCard (US) so that tuner could be the issue. I'm using LAV for all my decoders.
Hmmmm, after upgrading to W10 and putting server IP into hosts file, channel change now seems to work. I need to play with this more to determine if it the hosts file addition or W10. Whichever, very encouraging.
One minor issue is that there is a 1-2 second stutter after the channel change which I can live with.