When your files are finalizing that means that they have been successfully uploaded to our servers and are being committed to permanent storage. Your device does not need to remain online or connected for finalizing to proceed: it will complete even if you close SpiderOak or turn off your computer. If there are additional files to upload, those other files begin uploading immediately without waiting for previous transactions to finalize.
How long finalizing takes depends on the size of your transaction and whether or not there are a lot of users on your server cluster also trying to finalize at that moment. Slow finalization is nothing to worry about, however: those files have been received by our servers, and it does not affect your subsequent uploads.
That said, do let us know if finalizing of a small transaction takes more than a couple of hours. That could indicate an inefficiency in a server cluster that we would want to investigate.