I get 25% CPU usage for ts3client_win32.exe all the time on my computer running Windows Vista (SP2, Ultimate, 32bit, OS language Japanese) with quad core CPU Q9550.
(One core is always being used 100% by TS3 client)
My friend, who is hosting the server on Windows XP (SP3, Home, 32bit, OS lang. Japanese), also with quad core CPU Q6600 is getting same 25% on the client side.
Also, this is not the unique problem for Windows, but for Mac OS X 10.4.
My Mac has single cored PowerPC G5 processor experiencing high CPU usage. Roughly 60%.
I am clear that it's not something else like firewall anti-virus, etc. that's causing TS3 Client high CPU usage. We use 3.0.0-beta9 for the TS3 Client.
It seems like it's the problem with the client, but it must be a problem with the TS3 SERVER's banner feature.
I do not experience high CPU usage, unless the server I connect to have large banner set to it.
Connecting to TS3 official server creates no problem with CPU usage.
Also, connecting to server without large banner in new tab will decrease the CPU usage for the client, until you change the focus back, or close the tab.
My friend and I started running TS3 server from beta 1, experiencing high CPU usage on the day one on the client side.
First, we did not know the problem and of course, turning off firewall and anti-virus was the very first thing we did, which did not help.
Soon, we discovered that setting large Host Banner on the server would increase the CPU usage for TS3 CLIENT.
We tried with three different sized images (all .gif, non-animated). One is 240px×23px, 1024×50 and another is 1024×100. We had no problem with 240px one, but both 1024px images boosted CPU usage.
Here are the actual Banner Gfx URL I used on the server to set the banner.
Once you set the decent sized banner, erasing banner from server does not lower the CPU usage for the client until restart.
We tried hosting server on different Windows environment (XP & Vista), but the problem persists.
Now the server is running on Win XP (spec. stated on top) with 3.0.0-beta-11  but the problem persists.
Just realized that beta-12 came out on new year's eve. We'll try updating the server, but by looking at the change log, I don't think the problem is fixed.
Here is the link to the server experiencing high CPU, but this server is not up and running 24/7.