delays between chunks using cURL

Just for clarification once again: delay happens after the actual transmission of the data and waiting for the HTTP response (HTTP/1.1 200 OK etc.). I measure using 'time' from GNU coreutils alongside other linux specific tools and/or on our hardware firewall. Additionaly I use a paket filter to determine when the "HTTP/1.1 200 OK" request come back from Dropbox, and when it does, cURL exits. So the delay comes from waiting for this response.

Both, during simple file uploads and chunk uploads - the bigger the file/chunk, the longer the delay it seems. In the meantime I uploaded a file using python for testing and the delay seems to be present too.

I also checked in with a friend to check it out and he can confirm this (he uses the JAVA API), but he didn't pay attention before. Maybe other people experience the same but no one notices the delay, because they don't monitor traffic during the execution of their scripts and thus don't correlate the runtime of a script with the actual data transmission time? At least this is my guess.

Here are a few metrics for uploading into the same file into Dropbox but with different chunksizes (I've throttled the upload to constant ~50Mbit/s for testing purposes):

File Size      Chunksize           Total Transmission Time
100MB         5MB                     2:03 Minutes
100MB         20MB                  55 Seconds
100MB         40MB                   51 Seconds

But since the maximum possible chunk size is 150MB including a considerable delay between the chunks, the total time of delays during big file uploads is a HUGE hit. Here is another example including other providers offering HTTP API and using cURL and a fixed upload speed of ~50Mbit/s :

To                                Filesize           Chunksize             Total Transmission Time
Dropbox                       1000M           150M                     9:29 Minutes
Cloud Provider "X"      1000M              5M                      3:55 Minutes
Cloud Provider "Y"      1000M          222M                      3:49 Minutes

So while the transmission speed is the same during upload to different providers, the delays between chunks uploading to drop box almost adds to a double amount of time needed to finish the whole file.

Needless to say, this HUGE time overhead is not acceptable. The only thing I can explain this with is dropbox calculating a hash for the file server side and then returning a HTTP OK, which also explains the longer delays for bigger chunk sizes.