Why can't I get equal download and download speeds on a symmetrical channel?

I am assigned a project in which my code should download and upload some files on the same FTP or HTTP server at the same time. Speed ​​is measured, and some conclusions are drawn from it.

Now the problem is that with high-speed connections we get almost the expected results in terms of bandwidth, but with slow connections (I think the ideal CDMA 1xRTT link) either load or load winnings due to the opposite direction, I have a “higher body ", convinced that the CDMA 1xRTT connection is symmetrical, and therefore we should be able to transfer data at equivalent speeds (~ 100 kbit / s in each direction) via this link.

My measurements show that without heavy code tuning in terms of buffer sizes and throttling of the data channel, it is impossible to provide the same speeds in predetermined conditions. I tried both my multithreaded code and created a simple batch file that automates Windows ftp.exe to perform data transfer - the same result.

So, the question arises: is it really possible to perform data transfer on a slow symmetric channel with equivalent speeds? Is the “supreme organ" right in its expectations? If so, do you have any suggestions on what I should do with my code to achieve this throughput?

PS. I completely rewrote the question, so it would be obvious that it belongs to this site.