For me it is the other way around. when i speak spontaneously i usually have a really hard time to slow down my speech and control it. But when i read out loud i find it much easier to speak slowly and it gives me much better fluency. I think this is because when i´m reading i can just shut off my brain and not have to think about what i´m going to say. This might work the same way for you, only that you find it more difficult to read than to figure out what to say.