We’re seeing this issue on our Macs as well. This seems to be a an issue with macOS 10.13 where a macOS process called lsd (for Launch Services Daemon) starts acting weird after a while. That process is responsible for all kinds of things that have to do with files and apps on disk, like getting the names of apps (those can be localized) or determining which app should open a given file. LaunchBar uses official macOS APIs provided to developers to query exactly these things. Every time LaunchBar wants to show an app in the list, it asks the system for the name of that app and the system delegates this query to the lsd process. This is not something new in LaunchBar.
Recently (10.13.0? 10.13.1?) we started noticing the same thing as you did: LaunchBar gets slow sometimes. We found out that every time that is the case, lsd has high CPU usage. You can see this in Activity Monitor easily. Quitting and launching LaunchBar again doesn’t help, but killing lsd immediately makes things fast again (macOS restarts it automatically). But I wouldn’t recommend that because some things don’t work properly then (e.g. app icons disappear). Restarting your Mac solves the issue, too, and logging out and back in should also work (but I haven’t tried that).
Note that you can see the same problem without LaunchBar just by using Finder. When LaunchBar is slow like that because lsd hogs the CPU, just try the following: Open Activity Monitor and search for lsd. Open a Finder window and go to the Applications folder. Relaunch the Finder by clicking its icon in the Dock while holding the Control and Option keys, then choose “Relaunch”. When Finder relaunches, note that the window showing the Applications folder takes a long time to show anything, while lsd has high CPU usage. If you try the same thing after killing lsd, the window shows apps immediately.
I’m afraid I can’t offer a useful fix at the moment besides restarting your Mac. We’re investigating this further and hope that we can either do something about this ourselves or it will be fixed in an upcoming macOS release.