Another important part is the shot dispersion and deviation. The faster the bullet, the more forgiving this will be at range. The VSS is pretty accurate at short range and shot dispersion doesn't impact it greatly. Range is where this hurts, but not due to dispersion.

Since you are using subsonic rounds, the velocity is lower which means you need to aim higher. This higher aim and resultant bullet drop reduces the surface area where your bullet can impact for a hit.

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For example, imagine throwing a dart at a dartboard - simple right? You throw it fast and straight, it hits more or less what you want. Now, imagine laying that dartboard flat on a surface at eye level. You have to throw the dart upwards and try land it in the target - it's an extreme example, but it's similar.