Are you sure it is the lerping and not the rendering? Have you done any profiling?
I'm working on a procedural game that requires lots of unique mesh rendering and I found that once I got even a remotely large number of meshes generated the node system effectively killed my project.
I had to access the visual server manually and effectively had a few meshes where I would add surfaces to and from them. This switched my game from lagging from around 400 chunks (think Minecraft level of poly) to running fine on my integrated with over 16,000 chunks.
If you are rendering each hexagon as its own node this is worth considering.
If it is indeed the lerping itself that is causing all this slowdown, prefer storage over processing if at all possible. Even if you have to store a gradient of z values or something to adjust your hexagons by.
Also, use built-in functions (such as lerp) over calculating it yourself, as these are faster. I assume the math behind the functions get calculated native rather than having to be interpreted. That is just a guess, though.
If this isn't doable you could always pop the lerping onto another thread so it isn't lagging the rest of the game.