I noticed the same issue. Flying into Kamloops BC (CYKA) was using the terrain radar to navigate the mountains in low vis. As soon as the radar begins to detect terrain, there is a massive 20-25 FPS drop. Happens every time it detects terrain. Switched off the terrain radar and the FPS immediately returned to normal. Approach with terrain radar off = +30 FPS. Approach with radar on = 17 to 20 FPS. All other settings and conditions (weather, approach and so forth) the same. Only difference is terrain radar on/off.