When SpiderOak ONE or Groups is building a file for upload, it is making a temporary copy of it and preparing it to be backed up. This preparation is done in two steps.
First, it is compared to other files you have previously backed up to your account. This is deduplication and saves you bandwidth and account space. If your upload is only a little different from a previous version of a file, the build process will determine what those differences are and only upload the changes. This saves you yet more bandwidth and account space.
The second step is to encrypt your file. Before your file can be uploaded it is first encrypted with the keys that safeguard your account. This accounts for time and processor usage as your file must be encrypted before it leaves your computer, and encryption is processor intensive.
The time it takes to build a file for upload will vary depending on your hardware, the nature of the file, and how many files you have already backed up that will be compared to the file now being built. Most small text files will build almost instantly, but as files become larger more work needs to be done.