England's general problem is that they don't really beat any of the comparable nations in major tournaments. I can only really think of Germany in 2021 where they've beaten one of the top sides in world football in the knockouts at a major tournament (you could argue Spain in 1996 but Spain were rubbish in the 1990s).It depends on how you view underperformance , I mean there is different variations of it right ? Like there is one where you completely don't show up , get knocked out in the group stage like a France in 2002 , then there is one where you consistently put yourself/team in positions to win a trophy but for some reason or another you fall short ...I think this England side under Southgate falls under the latter ...
Ifs and Buts just don't cut it especially in tournaments that happen every four years . In Cricket we have India post Dhoni captain era who consistently make semi finals and finals of major tournaments but fall short of winning, I think there is a degree of underperformance to that .
Losing a quarter final to France is obviously disappointing but it's not an under-performance, particularly when this is exactly the sort of knockout game England tend to lose anyway.
Underperformance is the two tournaments immediately preceeding Southgate's appointment, ****ing up the group stage in 2010 resulting in a 2nd round tie vs Germany, not qualifying in the first place or being eliminated by teams like Portugal or Croatia.