This is normal and is good. Each paragraph of my first essay took at least 7 hours to complete and most of the time was spent thinking. By the way, I was not distracted during the 7+ hours.
Just keep at it and the brain will adapt. Your thoughts will become more organised and mental clarity will therefore improve as I've experienced. It was like most things were in order and I can search for what I want in my mind.
Slow thinking is not necessarily bad, it usually means your thinking has more depth. If you wish to become a faster thinker, then you just have to force out ideas quickly. It's like adding a little pressure (time constraint) so that your brain doesn't stay in it's comfort zone and think for ages.
But if you wish to becoming a better thinker overall, then I believe you should at least focus on introspection. That is, examining your own thinking. You'll be able to identify the cause of the slowness and fix it to your desire. Of course, this is not easy.