As your item 4. (and mentioned already), if you only need encrypted transmission, but are not worried about file storage at each end, just use scp or sftp, which most people should have anyway.
Also, I prob wouldn't worry about asymmetric performance from your descriptions.
As mentioned above, the "slow" bit can be the key creation, but that's a one off cost anyway, unless you intend to use a new one for each file (unlikely).
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