As far as I've seen, it's not. I used razor-qt when it was still an independent project and I pretty much fell in love with the idea of it. Gnome 3 was being a

As far as I've seen, it's not. I used razor-qt when it was still an independent project and I pretty much fell in love with the idea of it. Gnome 3 was being a PITA (I remember throwing things when my artfully crafted css broke with an upgrade) and KDE hadn't crossed over into version 5. I'd tried out XFCE at that point. It didn't excite me because I didn't want to get stuck with GTK 2. I just downloaded razor-qt and realised that with a bit of spit, shine and polish, this had the potential to become what LXDE was to Gnome. I'm pretty happy about LXQT emerging from the LXDE/razor-qt merger. Anyway, back to your question. razor-qt was pretty snappy on my machine, leading me to conclude that KDE was slow because it was KDE. Akonadi, slow animations, buggy widgets, you name it. I haven't tried KDE since then (this was back in 2012 or so), but I've heard that the latest incarnations are pretty snappy and light. Even if they aren't, I highly doubt LXQT will have any problems with lag.