Private scalars make no sense in OpenACC

[fspiga]

!$acc parallel loop private(itmp,gtau,gu) present(eigts1, eigts2, eigts3, g, mill,drhoc) copyin(u1,u2,u3,drc,fact,mu,gu0)

By default scalars are made local to the generated kernel. This makes them "private" (conceptually) and has the added benefit that these variables are more likely to be put into a registers. When you add a scalar to a private clause, you are creating an array of these scalars in global memory, where each loop iteration has its own element (gang or vector). Since the variable is now in global memory, your code slows down.