I do something similar with my electrical plans. The way I keep it somewhat speedy is to place all the symbols on a single tag and create a scene with only that tag on, then create another scene with everything else I need for the background on with the symbols tag turned off.
In Layout I then stack two model views, the lower one will be the background rendered as raster, the foreground model view will be the symbols rendered as vector. These tends to render fast enough for my purposes.
The experimental graphics engine may shift/distort native Layout objects a little but – as I understand it – if you export to PDF whilst enabled it exports as it would without the experimental engine on.