copying is the least of your worries, you should submit to be proud of a map (or, i hope its something you can be proud of...)
if units are spawned often using (center of region) or something like that, and you haven't cleaned location-pointers then those will build up and eventually cause lag, depending on how many are made over what span of time
just generally too many units can cause it to, but thats more just a graphics card not being able to handle the polys and stuff
i unno can't say much without details