Can’t say how it was for Endwalker, but I did experience a lot more server lag on Dynamis than I did before the launch. Getting in wasn’t the problem so much as being in instances and just having minute long multiple lag spikes to the point I was often going from lag to lag. Had this even in duty support. My understanding is this was “better” because I could even log in at all, but it did absolutely suck trying to learn new mechanics where I was in MSQ and learn how to play around entire dungeons being slide shows of lag at the same time.
ETA: I got up to Amaurot about the day after pre-order drop happened and then worked up through Seat of Sacrifice over the heaviest weeks was absolutely miserable with the lag to figure out what was going on. I also do MiNE ex/savage and had a few I’d been progging I just had to bow out from because when we got to new phases I was having so much lag they weren’t showing for me and the majority of the time I’d be either killed by something I never got an update to see or last person standing because I was lagging so much I didn’t ping into the damage.
Some of it was DDOS but a lot of it was just server overload which sucked horrifically. DDoS was its own thing because I did have trouble logging in during those times.
ETA: also not to sound like a churl the update about a month in did fix a lot of the lag. Until then it was just really bad and I was routinely running into people in df roulette also suffering from it which just made things even harder. But I will say SE did work on it and put a fix out that resolved most of it within a month. I feel like part of the challenge may have just been I was on the “overflow” datacenter for NA and SE underestimated how much “overflow” we would be seeing. Cause about the majority of people I was paired with in df roulette during this time were travelers which isn’t the norm on Dynamis ime.