Limitations of the TCP/IP protocol make it very difficult for a single application to saturate a network connection.
But S3 Browser Pro allows you to upload your files using network connection at the full speed.
How it works for large number of small files:
Pro Version

Pro version allows you to upload multiple files at the same time
Multiple files are uploading at the same time. Each TCP connection is working at the full speed. And in sum you are getting uploading speed you're paying for.

Screenshof from NetLimiter that shows how network connection is used by S3 Browser Pro
Free Version

Free version is limited up to the 2 simultaneous uploads
Only two files are uploading simultaneously. Free version allows up to the 2 concurrent uploads. For example you are paying for 2Mb uploading speed, but in fact each connection is limited by the protocol, let's say up to the 256 Kb. In this case you are using only 512 Kb of 2048 allowed.

Screenshof from NetLimiter that shows how network connection is used by S3 Browser Freeware
How it works for large files:
Recently, Amazon S3 introduced a new multipart upload feature.
You can now break your larger objects into chunks and upload a number of chunks in parallel. If the upload of a chunk fails, you can simply restart it. You'll be able to improve your overall upload speed by taking advantage of parallelism.
Multipart upload feature is enabled by default in S3 Browser Free and Pro. All files larger than 5 Mb will be uploaded via multiple parts in parallel.
But as you already know, Free version is limited up to the 2 simultaneous uploads.
Pro Version

Multipart Uploads: small parts of the large file are uploading simultaneously
Multiple parts are uploading at the same time.
You are getting full broadband speed.

Screenshof from NetLimiter that shows how network connection is used by S3 Browser Pro
Free Version

Multipart Uploads: free version is limited up to the 2 simultaneous uploads
Only two parts are uploading simultaneously. Free version allows up to the 2 concurrent uploads.

Screenshof from NetLimiter that shows how network connection is used by S3 Browser Freeware
To adjust the number of concurrent uploads:
Click Tools -> Options -> Queueing

Click Tools -> Options to show Options window
Configure the number of concurrent uploads and click OK.

Configure the number of concurrent uploads and click OK