@girish
UPDATE:
I installed s3cmd on the server and uploaded (s3cmd put) a 256MB file to the bucket. Got a consistent 15MB/s upload using the default 15MB chunk size.
Uploading a 1GB file, using the default chunk size, also a 15MB/s and with a 200MB chunk size, around 18MB/s
So, at 15MB/s a 100GB backup should take about 2 hours.
I don't know if installing s3cmd inside the container and testing from the inside would help.
I'll also check with linode if 15MB/s is what is expected as upload transfer rate.