retroarch - video_scaling gives poor performances

[etnbrd]

Hi,

I setup alarm and kodi on my Raspberry Pi 2 a few months ago. It is working well.
Yesterday, I wanted to play emulated games.
I installed the meta package libretro. Along, I had to install xorg-server and xf86-fbturbo-git as well.
(weird that kodi doesn't need xorg. Maybe it uses wayland ? I didn't figure that out)

The performances are just terrible, even on the menu.
By default, the video_scaling is 3.0, which results in about 2 FPS.
All 4 cpus are around 90% of usage, according to top.

When tuning video_scaling down to 1.0, I get around 12 FPS, which is not great, but fast enough to navigate the menu.
Then the cpus are around 60% of usage.

It should run smoothly : there are distros entirely dedicated to run retroarch on the raspberry pi.
So, what am I missing here ?
I don't know exactly where to start, any help appreciated.

Thanks.