I need to include a clip 1920x1080 to a composition 640x380. What is the best practice? Should I resize the original movie clip before by some external application before importing it?
I can resize it also inside of the composition by moving "handles", but am not sure if it is a right way to keep its quality.
When scaling an image down, you don't have to worry nearly as much about quality loss as you do when scaling up.
To scale an image the same amount both horizontally and vertically, just modify the Scale property directly in the Timeline panel with the Constrain Proportion switch selected, or Shift-drag a handle in the Composition panel. See "Scale or flip a layer".
One way to get a little bit of a performance boost is to scale the layer down and pre-render and replace that item. That way, you just have to do the scaling down once, rather than every time you render a preview.