You can configure Krunner to not use some services that may be causing tthe lag.
It's superfast in my machine.
Also, I think the first launch is the most problematic, so try to launch the background service at start, there must be some tutorials on this.
In any case, Krunner is not supposed to be a key-combo launcher. It's a search engine. Use shortcuts for opening your most used apps.
- Super+B for browser
- Super+E for file explorer
...and so on