I just used full image from sakaki latest build available here.
I got 25-27fps in fullscreen (4CPUs were at 100%) and it was still using lvmpipe.

I guess I will keep it headless.

Thank you both for the suggestions.

Jarodiv, you're right. This is why I upgraded to gcc 10.2 using distcc to avoid any kind of issue (I still know that I might find some weird issues):

I followed sakaki's guide Set Up Your Gentoo PC for Cross Compilation with crossdev

Except that I use eselect ...

Indeed, I have the same configuration.
Sorry, I wanted to avoid multiple replies to my own post so I edited my own messages.

Are you using the mesa package from Sakakisgenpi64 repo (with the "rpi-v3d" use flag enabled)?
Yes, I am.
I installed the package by manually downloading the ebuild and creating a custom local repo.
I get the following in fullscreen in 1920x1200.

$vblank_mode=0 glxgears -info

ATTENTION ...

Hello,

First, huge thanks to Sakaki and all contributors for maintaining and documenting the various guides on Gentoo. Especially for arm64.

I received the Raspberry Pi 4 8GB RAM last month, it works like a charm except for few minor things:
- I can't seem use 3D acceleration properly on arm64 but ...

Gentoo was the distribution I used the longest as an end-user when I was a teen (2003-2004). I broke it and fixed it so many times.
I was patching the kernel 2.6 at every minor release to be able to use the unsupported ECI USB modem.
Then, I stopped: I went to Uni and moved on with life and ...

./admin/cvs.sh: line 33: --version: command not found
*** AUTOCONF NOT FOUND!.
*** KDE requires autoconf 2.53 or newer
make[1]: *** [cvs] Erreur 1
make: *** [all] Erreur 2
I noticed that the error was produced by doing :
echo `$AUTOCONF --version | head -n 1`
bash: --version: command not found ...

Here is my emerge --info
Portage 2.0.53 (default-linux/amd64/2005.1, gcc-3.4.4, glibc-2.3.5-r3, 2.6.14-gentoo-r2 x86_64)
=================================================================
System uname: 2.6.14-gentoo-r2 x86_64 AMD Athlon(tm) 64 Processor 3200+
Gentoo Base System version 1.12.0_pre11 ...

I just found out what the problem is :
I had to do:
make -f admin/Makefile.common
but if anyone knows how to add it manually in the ebuild or have a patched ebuild,
don't hesitate to paste a link or what to do to get it.
btw, do you know where to find kdelibs-7-r3 ? I have only the r2 version in ...

I get an error with kdelibs-7-r2 :\
I have autoconf 2.59 installed but I get :

* Updating working copy of kde/branches/KDE/3.5/kdelibs recursively
SVN: At revision 487791.
* Copying files over ...
* Moving branches/KDE/3.5/kde-common/admin to /var/tmp/portage/kdelibs-7-r2/work/branches/KDE/3.5 ...