Sure, they had notoriously bad performance for a plurality of reasons. Which also explains the lack of specific complaints. People can tell you that it's slow, that doesn't mean they can tell you every reason why.
Firefox historically being slower than Chrome was for a long time one of main reasons cited by people who switched from Firefox to Chrome. DNS/proxy defaults were certainly not the only reason it was slower but all the little things add up.
And we shouldn't need to use user complaints to gauge performance impact when we can do so directly by measuring page load times etc.
