When using multiple (logical) cores for the build (/M switch), msbuild.exe spawns extra msbuild.exe processes (depending on the number of cores and build tasks). These spawned worker processes do not (immediately) shut down after build is finished, but they are re-used by subsequent invocations of msbuild.exe, so that's not a problem.
What is a problem though, is that the plugin hangs around for these worker processes to exit (I suppose), and with the 2 minute process termination waiting period introduced in 2.141, causes the build to stall for a period of a multiple of 2 minutes. As far as I can see, these worker processes are not a problem, and thus should not be forced to shut down. Setting -DSoftKillWaitSeconds=0 affects other things than this plugin, so I don't think that's a viable workaround.