First you can try disabling animations, i.e., setting the animation speed to ‘Instant’ (System Settings → Workspace Behavior → Animation Speed). Then the launcher will appear much quicker when you press ‘Start’ / the ‘Meta’ key.

Then you can try disabling some search plugins. To the right of the search field in the launcher, there’s a settings icon. Click on it and then on the ‘Configure Enabled Search Plugins…’ button. (The name of the button is misleading; you can also disable plugins here.) A good strategy is to first disable all plugins and see if this helps. If it does, enable each plugin you need, one by one, until the search becomes slow again.