Hi and welcome here 
Thank you for your interest in providing feedback.
For the problem of .deb packages…
Each distro has a pool of maintainers creating packages from the sources.
It is not Synfig’s team, it is independant, the choice to include any lib or version is theirs.
They maintain according the evolution of their version and environment, which permit to provide “expected” stability.
And as each distro is different…
…there is the appimage version 
Appimage permits to bring in a single package (mostly) all the required libs for the software, with the proper version.
As a drawback, it is bigger and need some install/decompression at startup.
The errors of fontconfig are known, it is because we use an older/more compatible version of the lib which doesn’t recognize new information format in the font file description, but no consequence on runtime.
Depending on your installation, you may need to set permissions on /tmp directory, use appimaged daemon…
If there is a problem of permission, the cache can’t be writen properly for example, it could explain why it needs so much time (recreation of font cache?)
And it depends if you use 1.2.2 or a later version (appimage format may be different)