No, this is actually not possible.
Im happy with it. I’m currently using my own webspace with webdav access, GDrive and OneDrive for Business. But actually cryptomator does not the sync of the encrypted files. so the “speed” of encrypting/unencrypting is regardless from your cloud provider. (at least when using the desctop app).
With the iOS app I have the highest speed with my own webdav connected space, followed by GDrive and the “slowest” (which isnt really slow) is the Onedrive 4 Business connection. But from my point of view this is also triggered by the provider (cloud and connection), not by the cryptomator app.
Please keep in mind that you do not open the files with the cryptomator app on mobile devices. You browse the file in the cryptomator app, and hand it over via “share” to the app that can handle the (unencrypted) file.