[petsc-dev] Lagging the numerical factorization for one of the fields of fieldsplit preconditioner

[Shri]

            | A  B |  |dx|   |fx|
 Jacobian =                =
            | C  D |  |dy|   |fy|

While solving this linear system with a fieldsplit precondioner,is there a way to lag the numerical factorization of the A matrix. The A matrix in my application is constant and is about 3-4 times bigger than the D matrix. The log summary shows that about 90% of the time is spent in the numerical factorization phase. So i would like to do the numerical factorization for A matrix just once and not have it factorized for all the snes iterations. 
Speaking on the same note, should snes lag_preconditioner functionality be moved to ksp or maybe a -fieldsplit_xxx_lag_preconditioner option?

Thanks,
Shri