You need to disable vsync like this when running glxgears:
vblank_mode=0 glxgears
For Nvidia drivers do this:
__GL_SYNC_TO_VBLANK=0 glxgears
Notice the higher than 59-60 fps results with vblank_mode=0:
ATTENTION: default value of option vblank_mode overridden by environment.
7919 frames in 5.0 seconds = 1583.704 FPS
8187 frames in 5.0 seconds = 1637.266 FPS........
When things go wrong your video is basically unplayable or the first video plays fine and then freezes when moving on to the next. Generally if both videos weren't produced with the exact 100% same settings you will have issues. You can try the basic concat but it often won't work right.
Solution for me:
My example uses 3 videos in total so "n=3" and a=1 to include audio.
ffmpeg -threads 12 -i file1.mp4 -........
#if you have nvidia make sure you install the nvidia-cuda-toolkit so hardware acceleration can be used
wget http://ffmpeg.org/releases/ffmpeg-3.3.2.tar.bz2
tar -jxvf ffmpeg-3.3.2.tar.bz2
cd ffmpeg-3.3.2/
./configure --disable-yasm
install prefix /usr/local
source path ........