Image Upload and manipulation API taking more time for higher resolution images

My Current flow that I am following for image upload and its manipulation on the S3 bucket is as follows:

1. Image is uploaded into an S3 Bucket.
2. Retrieving the image buffer of the saved image in the S3 Bucket.
3. Resizing of the image buffer using gm and imagemagick npm module.
4. Saving the resized image buffer with the original extension as a new image in the S3 Bucket.

As the image size increases so those the API response time.It takes 3 secs for the API to give a response while following above workflow for a 300 KB image and 30 secs for a 15 MB or more image.

I don't know if my flow is correct or not. My requirement is that I need to reduce my API response time while uploading a 15 or more MB image. Can somebody suggest me how to achieve this?