Nothing changes, unfortunately.
But for now I have been able to reach a decent speed, and I’ll live with that.
I had similar issues when I tried the latest plasma on unstable, initial opening of apps like dolphin and kate took roughly almost 2 seconds, and even after the initial launch sometimes when I would open them they would open almost instantly and other times they would take roughly almost 2 seconds again, I even tried installing to f2fs which is supposed to have fast app opening speed, but there was no difference.
So my work around was to install lxqt and use kwin as the window manager, if you don’t use any desktop widgets or special plasma features then lxqt with kwin functions essentially just like plasma, I made a custom lxqt display manager session so apps like openbox and pcmanfm-qt don’t get pulled in, the only issue is there is no desktop wallpaper because pcmanfm-qt isn’t installed, but I don’t care about that anyway because I always have the web browser open maximized, now opening dolphin and kate for the first time takes less than 1 second, and every time I open them now they’re consistently fast, opening almost instantly.
Lets hope plasma 6 fixes this issue.