Zone 8 where? And how do you define 'fast growing'? And to what mature size? And how near to the house? Sun or shade?

These are pretty critical elements to know before you can get meaningful answers.

Allen, Texas. Mostly sun, some shade. A few feet from the house in a flower bed. Looking for something that grows at least 10 feet tall.

Look at shrubs. Most will grow faster than trees and are more amenable to pruning should they exceed the desired height and width and will tend to be smaller than the vast majority of trees as well (only dwarf trees - and not many of them - will top out at 10'). They also tend to have less aggressively spreading root systems which makes them easier to incorporate with other, smaller plantings.

In specific, I'd look at some your Texas native shrubs. Most are fast growing and all well adapted to the climate. Plus, many are flowering shrubs as well.

10 ft. tall right next to the house? What is the issue, are you trying to block a view?

Japanese maples do well down there?

$$$ can replace the need for fast growing.

A few feet from the house in a flower bed.

A few feet from the house, you want to look for a shrub with a narrowish, vertical growing habit. If you ever need to paint the house, or tent for termites, or maintain the surface of the house in some fashion, be sure to plant the shrub enough away from the home so you can get behind it when it has grown. It will save you some extra work later.

Yes we are trying to block a window. For some reason the builders put a master closet window in front of the house.

"block a window"

A bit of hardware may be better than greenware.

Yes.

I don't think plants are your solution to this problem, get some curtains. You could plant a very small tree, or large shrub, like a dwarf crepe myrtle, or dwarf JM as mentioned by toronado.