Upload slow on S3 Bucket for large number of files

I am uploading large numbner of images on AWS s3 bucket via s3 api from my local machine. 800 images gets uploaded in 15 min, but 8000 images take close to 24 hours to upload. Each image is around 10 MB. The time taken to upload increases exponentially with larger number of files.

Transfer acceleration didn't help as I am close to the datacenter location. Multipart upload is recommended for mostly for larger files (>100 MB). What am I missing here ?

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