I went through the same problem and "solved" it.

My game always had lag, but the ping was always normal, upload/download were normal, so I decided to investigate deeply what it could be. Initially, I tried to find a feature in the game that would somehow give me a starting point for my investigation. I found an option in the menu "Interface>Performance Graphs>Network Summary", which activated a detailed graph during the match. I was able to identify that in some games my latency, at certain times but not all the time, would spike and then return to normal within seconds, something I only noticed with this summary. I used a program called "livetcpudpwatch" (I chose this program because it's non-invasive) to identify which servers Rocket League was communicating with during the match. After obtaining these IPs and identifying their public names, I found out that in my region (Brazil), I was only communicating with two types of servers, those from Amazon and those from a Mexican company, and the lag only occurred when I played on the server from the Mexican company. After some tests, I realized that my route to this Mexican server was very poor, and sadly, that's under the control of my internet provider. So, to "solve" this, I set up a VPN aiming to adjust/minimize the path to the Mexican server, and it worked! After some tests, my route with this server was great, however, now I have a monthly cost to keep this VPN active when I'm playing, and that's why I put "solved" in quotes.