is here a way to rebuild only onlinesubsystem of engine?
whole engine project even without Programs folder in VS rebuilds too long, more then 30 mins on my 4 core i5-4570 (3.20 GHz) and 16 GM ram
If you only edited some onlinesub files you should just be able to right click on UE4 module and hit Build, and it will only build changed files/modules. Hitting rebuild will clean all the binaries and rebuild the engine.
your method work only for projects, seems onlinesubsystem isn’t independent project, just a folder with .cpp and .h files of a whole engine 
located at
source -> runtime -> online -> OnlineSubsystem%
TheAgent is correct, happyhorror what you are seeing is the VS project, it doesn’t look like individual projects but UE4 has a custom build system: if you only change the onlinesubsysstem then only the dependencies will be rebuilt. Keep in mind that this might require other projects to be compiled if you change public headers. I do this all the time and can vouch that only the dependencies are rebuilt, not the full engine.