You can upres guides in Houdini with minimal performance impact using some of the hair nodes:

KK_hair_upres.hiplc

The naive way to make high-density hair in Houdini would be roughly the following:

1. Copy a bunch of lines to a surface (the density = the output resolution)
2. Sim all of the hairs at once with a vellum solver.

This method will work; however, with lots of geometry/strands of hair, the simulation can slow down significantly and be difficult to iterate. You may need to use more substeps in the Vellum solver to deal with the large amount of geometry and possible intersections.

BUT, (as it always seems in Houdini), there is a better approach to upres vellum hair simulations:

1. Copy a few guide lines on a surface
2. Sim just these guide lines
3. Copy lots of lines around these guide lines that mimic the motion of the original guide line

This method has the same hair resolution as the previous naive vellum sim but is far lighter because the vellum solver only has to work on a few guide lines.