And it finally happened. I kept using krita 2.8 as it was possible, but now my OS upgraded (From Ubuntu 14.04 to Ubuntu 16.04) and krita 2.9 is the new default release. 2.9 is awesome, but the freehand stroke tool really slow, making it unusable. I noticed that is a common issue for users with AMD graphics, and it is suggested to "disable vector optimization" or something like that. But in my window of krita performance (Preferences -> Configure Krita -> Performance) I don't find any tick to put or remove on "vector optimization". Am I searching in the wrong menu? Anyway, there are other ways to make krita 2.9 less slow?
Here the details of my desktop:

Ubuntu 16.04 LTS 32-bit
RAM: 2.0 GiB
Processor: Intel® Core™2 Duo CPU E6750 @ 2.66GHz × 2
Graphics: Gallium 0.4 on AMD CEDAR (DRM 2.43.0, LLVM 3.8.0)

Thank you in advance

That's because Ubuntu's Krita is still too old, it's 2.9.7 iirc, and we're at 2.9.11 (and almost 3.0 beta).

So, do you suggest me to upgrade it via dimula73 ppa?

You can do that, certainly recommendable for 2.9, what is also possible is to try the 3.0 appimages available from the news posts: these are portable builds that work on most linux distros.