Have you downloaded and read the free grow bible?
A lot of your questions might be answered in there. The light from a window seal might not be enough for many strains to develop healthy. 900 lumens might not be enough either in most cases.
Also I’m not sure what you mean by speed up the process? You can use regular fluorescent lights to grow a plant, but you need enough light and the right spectra. A whole lot of light can speed up how fast and big a plant gets, but it won’t necessarily speed up the whole process from seed to harvest, no matter the intensity or quality of light, the life span of a plant will just take its time. To get the plant to flower sooner, you can do that by manipulating the light versus dark period schedule, all that is also explained well in the grow bible and elsewhere here on the blog and in the forum.
Generally, seedlings and clones require about 400-1000 lumens per square foot.
Vegetative growth requires about a minimum of 2,000 to 3,000 lumens per square foot.
Flowering requires about 5,000 to 10,000 lumens per square foot, ideally, and can take possibly much more.
10,000 lumens is about the average power of the sun at sea level on a clear day at high noon.