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MyNameIsMrBurns - On Windows or Linux you could move metadata.db to your ssd and put a symlink to it in the library folder, but I'm not sure it will improve startup times by as much as you're hoping.
As a test, you could copy the metadata.db to an empty folder on your ssd, and then point calibre at the folder via Switch/Create library and tell it the folder is an existing library - the author/book folders are not mandatory. Then see what startup times are when it is the current library
Here's as 'how to' for Windows & Linux symlinks ==>>
Complete Guide to Symbolic Links (symlinks) on Windows or Linux
Apple fans have told me that Symlinks dont work on OS/X, not having a MAC I've no idea if that's true or just an urban myth.
If you do decide to keep the metadata on the ssd, then you'll need to ensure it gets backed up with the rest of the library.
BR