Thank you for your message.
We would recommend you to use the Dropbox feature Smart Sync :https://help.dropbox.com/en-en/installs-integrations/sync-uploads/smart-sync
This way you could choose between putting your data locally or "online only" (which would free your local space).
You can watch the short video in the link above, it will explain to you how you can use the feature.
Regarding the Boxcryptor encryption process, there is no sync time required for Boxcryptor to encrypt your data, this happens instantaneous.