Your trees are bright because you removed it from ray tracing, so its not calculating where to get its shadow details from.
What you can do is use HDRI, and then you won’t have problem with it but if its an open environment like when your camera or character moves around then your HDRI wont look good.
Another thing which you can do is select your Directional light and change your shadow method from ray tracing to disable, it would be less accurate shadows but it will solve your performance issue.
Also check if the megalights is using raytracing or virtual shadows, choosing between these two can also vary in performance.