When you apply upload latency, it also affects download performance because of how TCP (Transmission Control Protocol) works.
In a TCP connection, downloading data isn't just about receiving packets from the server. It also involves sending acknowledgements (ACKs) back to confirm the successful receipt of data.
So, if there is increased latency on the upload path, these ACKs take longer to reach the server. As a result, the server slows down its transmission of new data because it waits for ACKs before sending more packets (a process known as TCP flow control). This delay in ACKs causes a reduction in overall download speed.