[xubuntu] Ruing updates after fresh install greatly slows down browsing / flash videos

I have an old laptop which I only use to watch youtube videos while I am waiting for some process to finish on my main computer.

its an Intel Pentium M 1.8 Ghz, 1GB RAM Toshiba laptop. I have installed Xubuntu 15.04 on it.

Now the problem is that after a fresh install, the system works perfectly. Youtube and flash videos run smoothly, without any jitter or problems. But as soon as I run the updates, browsing becomes slow as anything and it becomes impossible to watch Youtube or any online videos. The videos jitter a lot and video freezes while I can hear the sound.

This has happened twice now. The first time I though it must be the extra apps I installed that must be causing this. So I did a fresh install and only ran the updates. As soon as the updates finished Youtube started acting up again.

So I formatted the drive and did a fresh install again. And have been watching Youtube all day without any problems.

Why does updating cause this?

Is there any reason or workaround for this? Or is it best that I just disable all updates?

Thanks