I've always thought it was a real bargain to put a relatively cheap stamp on something that purchases a service where someone else drives to the end of my driveway, picks up something, carries it across the country for me and puts it within a few steps of the recipient's grasp. Not always fast, but we didn't always have smartphones or email, so it seemed okay.
It's amazing they handle such a huge volume of mail and get most of it right. However, I receive incorrect mail weekly, sometimes I drive it down the highway to the correct owner because it was misdelivered to me a second time after I put it back in the mailbox with a note. I also often receive torn pieces of mail, and some things with special labels attached that explain why it was delayed months. That's my experience, and I still use it's service...it's cheap. But I was relieved when I could pay bills online and not worry whether my check got to the electric company, etc.
Of course the USPS receives financial benefit from the US government (i.e. taxpayers), including billions borrowed annually at favorable rates for its cash flow, and it doesn't pay taxes like its competitors. It is a monopoly, with protected markets, wages and job protections that are not competitive in its industry.
So I haven't noticed it getting any slower, but don't have high expectations for fast. I have noticed things ordered online getting slower...still waiting on an Amazon Prime order from 2 weeks ago.