The content quality has gone down as well but I can't ever say reddit was known for high quality content.
Also the fact it has so many users has caused its decline. Reddit today has so many obvious shill/advert posts.
I think that is to ensure that when they tell you "it's faster to use via our app" that they're telling the truth; certainly when the 3 "use app" buttons per mobile view came along mobile appeared to get slower (the change also including cunning "continue" confusion to make people think they're continuing to use the website but they're really "continuing to nothing and instead changing your mind and installing our app").
When it first began. before there were subreddits, it was a place for respectful, serious conversation.
A few years later /r/TrueReddit was started as an attempt to get back to the glory years. Now it's as bad as the rest of reddit.