The natural thinking style of Ni is to make leaps, from one idea to the next to the next which might be related to the first idea and so you loop back and jump off on a different tangent. What you get there resembles a rather messy spider's web which is focused upon your first idea.
Just because each leap makes perfect sense to you doesn't mean the connections are readily explained to somebody else. Therefore in order to explain, you have to start from the center of the web and pick 1 path to follow.
Therefore in your mind, you have to do two tasks simultaneously. One is to keep Ni (spider) spinning its web. The second is to plot a reasonably direct approach through the web to reach the fly.
I don't know about you, but my mind can't attend to 2 things at once. I switch back and forth between the two. Which means, in order to plot the course I must remember which spot Ni left off at. (Hence holding onto the train of thought) I don't mean to stop it. I mean to temporary pause it. Find your words, then let Ni move on. Repeat.
Faster is not good. The faster you go, the more your natural tendencies take over. Which means N overruns everything. N is anything but systematic.
Slower deliberate thoughts are better.