I'm probably not the best guy to answer this, because the 830 was my very first Windows phone and I returned it after a few days due to a bad experience. But here goes-
1.) Yes, TONS of delay and lag. This was the main reason why I returned it. My primary phone is a 2014 Moto X, and it is blazing fast, super fluid, and completely lag free. Compared to the X, the 830 felt like a 2-3 year old phone. Apps took a lot longer to load, the UI was more than occasionally jerky, and I became very acquainted with the "resuming" screen. I can understand getting that screen if I'm doing a bunch of multitasking and switching between apps, but I saw it most when returning to the homescreen, and that is unacceptable to me. The keyboard was also horribly laggy at times, often multiple words behind my typing. In all fairness, it was primarily in certain apps, Tapatalk being the biggest offender, and it was not laggy at all when typing in system apps like messaging. The Nokia camera app also seemed to take forever to load. There was a second camera app built in that opened much quicker, but I think the Nokia one had more features so that was the one I used. Also, there was major touch input lag, the worst I have ever experienced on a smartphone. I could move my finger a few millimeters without the screen responding. Aside from the Moto X, I also have a Nexus 5 and an iPhone 6, and the 830 just couldn't hold a candle to any of them. I do realize that the 830 is a midrange phone and just doesn't have the specs that my other phones have, but I was under the impression that Windows phone didn't need the latest and greatest specs to run great. Well whoever told me that lied.
2.) I've never had another Windows phone so I can't answer that.
3.)Sound quality was fine. I didn't listen to music that much in the few days that I had it, but when I did I had no complaints about the sound quality. I will say this though- call quality was absolutely superb and was at least on par with my Moto X. People sounded crystal clear, and said the same about me. This is one area where I was definitely impressed.
4.) Aside from the lag and constant resuming screens, I didn't notice any dealbreaking bugs, but again, I only had it for a few days. My phone never randomly rebooted like some people are experiencing, and the build quality was absolutely phenomenal and was one of the things that sold me on the phone in the first place. It looks great, feels super solid in the hand, and the curved glass screen is a nice touch (the Moto X has a similar curved glass screen).
Overall it was a very disheartening experience for me. I am very unbiased when it comes to smartphones, and was really looking forward to adding Windows phone to my collection. I had waited a long time for a high quality WP device to come to AT&T (I really wanted the 930) and I was super excited to get the 830 when it came out. Maybe I set my expectations too high, because ultimately I was incredibly let down. I'm not giving up on Windows phone just yet, but I will wait for a proper flagship to come around before I pull the trigger on it again. So all this stuff said, I unfortunately cannot recommend this phone.