I've never heard that about San Diego - that's interesting.
There is another usage in the United States. A slow death can refer to anything that is agonizing, painful, and something we just can't get away from. We may sometimes refer to that as dying a slow death. (This is not used that often, and when it is it is used for dramatic effect).
Any time anything fades away gradually, especially if it is a painful fading, we may say it is dying a slow death.
When we use it in the US, the emphasis is usually on the stress or pain of the situation as it deteriorates.