Each workspace has "cache" files, which are normally calculated and maintained by the Designer.
When these files are missing or broken, the Designer needs to calculate them again and it can quite some time on big workspaces (several minutes for very big workspaces).
Once this is done, the next time the Designer opens, it will not be so long : a few seconds to not more than a minute, I would say.
If you are stuck with this workspace feel free to share it with the support team (ftp server), to see what is wrong with it