Mirror Rendering with Planar Reflection drops the FPS

Hi, is it possible to improve the performance of FPS using an SLI solution? I have a room with 3 PR and the client wants the most detailed reflections as possible. A more powerful graphics card (Titan V)?
Now I mounted a 1080 ti and without the PR runs at 60/70/100 FPS, activating the PR drops to 15 FPS (minimum value framing the PR).