std::thread runs A LOT slower than std::future

I have some simple rendering program with a Mainloop that runs at about 8000 fps on one thread (it does nothing except draw a background) and I wanted to see if another thread rendering would upset the current context without changing it (it didn't to my surprise). I achieved this with this simple code here,

m_Thread = std::thread(Mainloop);
m_Thread.join();

and this code here somehow ran extremely slow, ~30 FPS. I thought this was weird and I remembered in another project I used std::future for a similar performance-based reason. So I then tried it with std::future using the following code:

m_Future = std::async(std::launch::async, Mainloop);
m_Future.get();

and this runs just a tiny bit below the single-threaded performance (~7900) fps. Why is std::thread so much slower than std::future?