> When writing single-threaded code takes 90% of effort of writing multi-threaded one

That "when" is doing some heavy lifting! More seriously: You raise a very interesting point. When I moved from C++ to Java (10+ years ago), I was initially so nervous to add threads to my Java code. Why? Because it was (then) difficult and dangerous to do it in C++. C++ debuggers were awful, so I didn't think I could debug problems with multi-threaded C++ code. (Of course, the C++ ecosystem has drastically improved in the last 10 years, so I am sure it is now much more pleasant (and safe) to write multi-threaded C++ code.) When I finally sat down to add threads to some Java code, I could not believe how easy it was, including debugging. As a result, going forward, I was much more likely to add threads to my Java... or even start with a multi-threaded design, even if there is only a modest performance improvement.