This is exactly also my story. Was a long term XFCE user (this was long before lxde became popular) because Gnome/KDE felt too heavy for my old computers. These days, KDE still has the silly loader window (no other DM has it) but oh boy the features you get once it is running are outstanding.
This is not only plasma, but all the applications are top-notch quality. Just to name a few: Krita, Kate, the office suite.
IME[0], that's only true for the X11 session, which is snappy, while the Wayland flavor feels sluggish and uses more CPU. Some distros are switching to Wayland by default so might be worth trying both and see what runs best on your machine.
[0]: Possibly hardware-related? Older Intel mobile 4c here.
Another echo here. Xfce was my beloved desktop until Gtk 3 started transforming it with design elements that hate me. Plasma is my new home, and after some tweaks, I'm pretty happy with it.
This is not only plasma, but all the applications are top-notch quality. Just to name a few: Krita, Kate, the office suite.