How can I do a gradient height tweak for a Solid layer?

For instance I have Solid A and Solid B doing height blend. How can I add a subtle gradient to solid B to sink it beneath Solid A at one side ?

I see gradients in mask stack but how can I multiply that gradient over layer height?

Also why a height blend of just two layers in 4096 texture is so slow and unresponsive ? Does it depend on image resolution I drop to solid layers?