Concurrent builds have been recently enabled on my system, and I noticed that Cppcheck plugin is having issues dealing with this new scenario.
Builds are running in different machines, and are not dependent on each other. However Cppcheck will only continue once they are all completed. This causes a delay of up to a few hours, where the first build will only finish once the last that is currently building has finished as well.
From the console output:
That seems to be a known bug for plugins written before this Jenkins feature was implemented, some of which had to be modified to address concurrent builds properly.