Every add-on like WLVFR or Power Lines adds a load on the CPU, so you can get over the edge and get stutters. Hopefully, the next version of WLVFR will be better optimized, as some of the models could be made more cpu friendly (especially the construction cranes).
I was asking about FSbirds specifically because it looks like they’re adding tons of objects with animations which is even worse for CPU load.