I cannot really comment on the GDI/Lightworks side of things, I currently use 2018 with the RedSDK plug-in and my current drawing is 289MB (yep it really is that big and is starting to slow down just a bit).
What specifically is the problem. i.e. What sort of drawing (all 2d or partly 3D). Is it slow at moving things in render mode (this is normal for me in Lightworks). Is wireframe OK, - wireframe should be able to handle that file size without problem. are there any large (file size) images in the drawing. using 3d, do you have many rendered viewports, things like realistic bolts/threads, and springs can slow down TC quite a bit (on my computer).