c++ 11多线程比单线程慢

I'm absolute beginner in multitasking who read some basics and try to get it to my project for object visualization. The problem is that my multithreaded solution which I implemented is slower than single threaded one and I don't know why and I have unexpected application code for unknown reason. I give you two cases where I tried to achieve better performance. I would like to know what I don't understand and where I have mistakes in general point of view. I give you part of source code and summarize all questions at the end. I'm absolute beginner in multitasking who read