The response header shows, that nginx is used already.
There is no need to put static assets to some cheap vps if you have already a server. Depending on the bootleneck, it might be much better to upgrade ram or optimise the code or finetune some database cache or whatever happens at the BE.
But without knowing details, it doesn't make much sense to move assets or use memcache. Often enough, people do shitty sqls or are not even aware that there is something like a query cache (which you can also use lightweight without adding memcache to your stack).
When you look at there about page and also make a lookup on there ip, you will see that it is hosted by them from there own network https://www.inria.fr/. So this project was probably build by some researchers, which some agenda on how well that service should actually run and a budget. They probably got a VM from there IT Department (or are part of it) and are in theory capable of fixing there issues but the working student is on a new project and no one really cares about it (the domain was registered 2014).