I'm facing the exact same issue, I'm on Kubuntu 18.04 and Google Chrome 68.0.3440.84 (Official Build) (64-bit)
I have the "binary extension" installed (to upload files), not sure if it's relevant.
This is a show-stopper for me, it's making lastpass completely unusable on my work computer (I am a software developer and as such use a Ubuntu based laptop).
I'd love some guidance on how to debug it.
Basically any "copy" functionality (copy password, copy username and copy generated password) ends up in a huge delay in the browser.
In addition, the keyboard stop working in all windows except the terminal.
If I kill chrome, everything goes back to normal, I can reopen the browser and everything is smooth.
I don't know if there is some sort of new bug in X that might be causing the problem, but if I use the github button "copy" (to copy the repository URL), I don't have such problem.
I believe the issue is caused by how LastPass extension is behaving when using the "copy to clipboard" functionality.
This problem happened only in the last 2 weeks, but it's impacting my work drammatically.