So like the title says, converting videos either slows my internet (just on my computer!) down to a crawl, or it doesn't work at all! I'm running Ubuntu 10.10 with all the latest updates!
I've never worked with Ubuntu or any linux more than the occasional using of a linux cd to do an outside of Windows repair to a Windows OS. I sounds like the conversion software you are using is using almost if not all the CPU that is available. With Windows I use PC Wizard 2010 to monitor CPU use.When minimized it shows the activity on a small see through screen. I know when I use DVDRebuilder it uses almost all that both cores have usually 95 to 97 % on both. I usually don't use the Internet during encoding but I have tested while encoding just to see what happens & the result is much like you describe. So if there is a CPU monitoring software that works with Ubuntu I suggest giving it a try during the conversion process.
The problem with most linux distro's is the video card drivers. So, depending on which card you have (AMD or Nvida) check over at Phoronix for driver benchmarks. There are some open source drivers and also some proprietary drivers to try. You don't give any hardware specs so no telling if it's memory, cpu, or video card related. Also Ubuntu doesn't come with a lot of codecs for audio/video, did you install any? Linux Mint is a bit more user friendly where multimedia tasks are concerned. What program are you using for the conversion? Which browser do you use?
I'm using avidemux, I have an nvidia 8600 GTS, 4GB RAM, 2.4 Ghz 64 bit AMD CPU. anything else you need? I DID fix the problem though, letting converter run on one CPU core, leaving the other free for other tasks!
Looks like you found a solution. I was going to suggest trying Handbrake and/or Winff for the video conversion, Handbrake only outputs to MP4 or MKV. It's odd though, I've not experienced any real browsing slow down when using Avidemux and I'm using a AMD 3800 with 2GB ram, but I do use Opera as my browser, FF always seemed like a memory hog to me. I also use Ubuntu 10.10 with an Nvidia 7900 GT and proprietary drivers.
I decided to convert a video with avidemux . I have it set to high priority & 2 cores .It has ran up to 100% on both cores during the conversion. So I think RCM's solution of using one core for conversion is a good solution. All it should do is make the conversion slower.I'm using Windows VistaHP . PC Wizard to monitor the CPU use.My processor is a : AMD Athlon(tm) 64 X2 Dual Core Processor 3800+ 4GB RAB but with Vista 32 only 3GB usable. I can still surf but it is slower when encoding with avidemux.