I'm slowing down my pace to really finish this one. I'm still happy with the way it's going, but I'm at the point where the changes are subtle and so the advances seem minimal. This is a tough stage also because I can really mess it up my overworking it. That being said, considering the size and complexity of this painting, I'm proud of myself for accomplishing so much.
Now, a quick word about painting speed. My natural painting tempo is very fast and that has a number of benefits; it definitely helps when I'm on a commission schedule, but it also allows me to improve rapidly. I'm not saying that there's anything wrong if you're a slower painter, but your turnover won't be as good as mine. Everyone has their own tempo so you shouldn't feel pressured to go too fast or too slow. The key is going the right speed.
Painting also isn't always a linear course to completion. The best paintings can go backwards for days before hitting a breakthrough. I will say that your tempo shouldn't feel unnatural. The begin is often times fast, but the end is usually slow and painful. For me, I personally have a burst of energy at the beginning which tends to dissipate as I reach the end. And then there's always the problem of when is the painting finished?
I try to finish when I'm at 95%. I know it's tough to quantify it, but I always leave a little room so that I can go back if I want. Problems arise when you push, push, push and overwork it and can't get that spontaneity back. If you're painting an eye, it may be the best it can be after 10 minutes of work, but you can keep pushing and it'll all fall apart.
Never overwork and think about how you want the final image to feel.