Hi Mappak
Due to the way that HD Terrain works (using Single Pass Terrain rendering, and an entirely new 'LOD' system), generally you should see similar performance on HD Terrain when using the same performance settings as before. However we have seen some reports from some users that they are seeing lower performance in some cases, which we are currently working to gather data on.
If you create a clone of your route you can simply test the HD Terrain to see how it performs on your computer. If you only see a small change to performance, then you may be able to account for this by adjusting some settings (some users have reported that lowering the anti-aliasing a little can help here).
Running Rysen9 3900x with rtx4060 and 64gb ram. Some areas bog my computer down while others run okay. Planning to up to rtx 4070 as soon as I can afford one.