WP8 lags under the following circumstances: you've filled the storage to such an extent that you have literally a few hundred MB (or less) left. Then it starts to display more issues. That would be the same on any OS on any device. Same for your PC. Try running your home computer when you have the hard disk basically full.
As for me...coming up to 2 weeks with the device. Enjoying it a lot. Purposefully not filled it with crap. Having the less storage actually makes me consider each app for quality and usefulness. If I install one and don't like it...instant uninstall. I imagine with extra storage people would tend to just leave junk apps they never really use. Keeping everything to a minimum helps keep it all speedy and smooth too. After a few charge cycles and working out my usage, I comfortably get about 60 hours on a charge which is such a plus for me. One thing that truly irritated me about smart phones was the crappy battery life. Seen a fair share of iPhone users talking about getting 24 hours as if it was some sort of witchcraft...lol. As mentioned in my original review I had a "feature phone" right up to 2012 and it would last up to a week. To get a budget device that excels in this department is nice.
As another comment mentioned, Huawei have basically turned this into the WP8 hackers phone of choice as apparently it's totally open for modification and loader changes. Not investigated that yet.