Green screen video issue

i’m sorta new to shotcut i’'m having issue with the green screening or a video layered over another in general it seems to slow the frame rate of both videos down and i’m not sure why and i’m not really sure what i can try if anyone can help please do.

(i have the latest version)

It is slowing down in preview because it cannot do all of the work fast enough for real time. You need to tolerate the preview lag and observe that the Export result is fine.