I don't mean to call you out, but the size of the map does not create lag, so if that is your sole purpose for creating this thread then its a bit pointless.
You may be getting confused with the lag caused by saving. This is not because of the size of the map, it is because of how minecraft stores chunk. At the moment each chunk has its own 4kb file. This means there are literally 100's of thousands of tiny files for the world. The 1.3 update is bringin a new map format which groups chunks into 4mb files. This means the save lag should almost completely go away, which is a reason NOT to get a new build map tbh.
People also may wonder why PVP does not lag when saving, when the build server does. The PVP server is on a ramdisk - an insanely fast hard drive. There is not enough room on the ramdisk for even a small build map, so the build server is on a Velociraptor mechanical hard drive, which is a lot slower in small read/writes, i.e. the 4kb files in minecraft. A new SSD has been ordered which the build server is going on. This should happen within the week.