it's nothing about the "soul"
you need to read the instruction
first, what is you computer hardware level?
if your computer is old and don't have large ram...
which kind of footage? HD OR SD?
when you are working, adjust the preview window
quality to low level( 1/2,1/3,1/4)
select the layer, right click,and press the "quality" to "Draft"
or use your video card to render, select "openGL"(IF YOUR VIDEO
CARD IS POWERFUL)
that will be help
or not, please upgrade your computer