Sometimes things don't go as plan.
If you are doing NaNo trying to achieve 50k in a month, and not faring well, I would suggest you to not think about it but let it flow, write it out. When you came to the end, you may realize that your subplot is not too early but too late to start. After you finish the first draft, then you have a much better idea how you want to weigh your plot importance. Whether is that MAIN plot is as main/interesting as you first plan? That is where editing comes from.

And if you are doing well with word count and have plenty of time, then you could look into your planning again.