web server – Understanding and changing of TCP priority

There is a busy web-server that has too many visitors, most of the time, it takes lot of time to place my order in that website.
First of all, it is not clear for me that how the web-server decides what packet to process first and in what order the others would be processed?

and my other question is that ,is it some how possible to manipulate packets with some tools like Scapy to have a better priority ?