Hi, it's normal that render times increase a bit with Forest. Usually an unique FP object may add many instances to the scene, sometimes using complex materials.
But if that extra time is normal or not, it's difficult to know: depends how long, materials, render settings and more.
Please, would you do a test converting your objects to instances using Forest Tools
If problems persists, i suggest you ask in the ChaosGroup forum. They have much more experience than us, regarding V-Ray settings and optimization.