This usually points to a mechanical drag issue in the focal-plane shutter rather than a problem with the speed setting itself. On a Speed Graphic, the rear focal-plane shutter uses a spring-driven curtain moving between rollers. If shutter timing changes only when the camera is turned sideways, gravity is likely affecting friction somewhere in that mechanism.

Most likely causes are:

• dry or sticky roller bearings/bushings
• curtain rubbing somewhere in its travel path
• aging, stiff, or brittle curtain material
• weakened spring tension from age

Because the shutter works differently depending on orientation, the mechanism is probably right on the edge: upright and upside down it runs acceptably, but sideways the extra drag slows it down.

What to check:

• inspect the curtain path for rubbing or misalignment
• check rollers for smooth movement
• look for hardened lubricant or dryness at the roller bearings
• inspect the curtain for stiffness, cracking, or deformation

The practical fix is a careful clean/lubrication and inspection of the shutter rollers and bearings, plus checking spring condition and curtain condition. If the curtain is brittle or the spring has weakened, parts may need repair or replacement.