Yes, of course, there are bigger issues, but technical performance and scalability is a significant one. It's not the lagginess that's the bigger concern, it's the scalability. Right now, to the end user, it's fine - there are only a few thousand active users, and you only need to have a few SP to be active. But imagine if it goes up to hundreds of millions levels of activity - it'll cost a pretty penny just to make votes and comments. Not to mention, even after MIRA and other improvements, and despite very limited activity only 3.5 years into this, it still takes an entire week to replay a bare minimum Steem blockchain node. Who knows how many weeks it'll take for a full node. You can see how scalability is a very real challenge that makes social networks ill-suited to blockchain.