Unfortunately our example isn't representative, and avoids a lot of things that make eigen slow to work with. Parsing the header file will take a constant time,

Unfortunately our example isn't representative, and avoids a lot of things that make eigen slow to work with. Parsing the header file will take a constant time, but template expansion etc. can take a long time if you have more code. Try adding a bunch of matrix operations (particularly chained ones resulting in massive expression templates), fixed size matrices of various sizes, and compile with optimisation and SIMD enabled, and you'll see different results. I really like eigen and use it a lot, but it can definitely result in long compile times.