250kts isn't fast enough for minimum clean speed in a heavy-ish 747, we need to go faster to climb properly. We'll often get a "No speed retrictions' climb below 10,000', and accelerate to 300kts (or more) to 10,000' then settle on the normal cruise climb which is usually between 320kts and 350kts.
For approach at max weight (Classic at least) We need at least 240kts clean.