Desktop briefly becomes unresponsive when typing on 2 keyboards at the same time

I'm running Ubuntu 18.04 and Gnome 3.28.1 with all the latest updates from the repos.

If I connect 2 (or more) keyboards to my computer and type on both at the same time, the desktop briefly becomes unresponsive. More keypresses increase the time of unresponsiveness.

E.g. when typing a full sentence in gedit or the terminal, only very few characters show up on the screen (I guess up until something is pressed on both keyboards at the same time), then everything freezes. Then after a while (some seconds), all the characters show up. And the desktop becomes responsive again.

Symptoms: During the freeze, windows stop updating their content. If seconds are enabled on the clock at the top middle, these freeze too. But I'm still able to move the mouse pointer around during the freeze without any problems.

The back story:

I got an ergonomic keyboard (R-Go Split Keyboard) yesterday, and quickly noticed the issue. But initiallly I thought it was an issue with the keyboard, and the problem wasn't that bad. But as I've gotten more used to the keyboard since yesterday and started picking up a proper typing speed, things got worse.

The keyboard is technically two separate cabled USB keyboards each with only about half of they keys of a normal keyboard (or at least that's very much my impression).

After getting the suspicion that this was a software issue, not a hardware issue, I tried typing on my old Logitech keyboard (Unifying Receiver) along with one of the R-Go halves: Same issue. Then I tried each of the halves without the other (as in "single" keyboard typing): No issue. Then I connected another Logitech keyboard (separate UR), typed on both Logitechs: Same issue.

Some further observations:

1) Nothing of significance in my syslog.
2) If I drop to a non-graphical shell, there's no problem. I suspect this issue is related to X.org or Gnome, but I'm in no position to say anything credible about that.
3) Also no problems when connecting the R-Go keyboard to a Windows or a MacOS machine.

Let me know if there are any logs I can provide or things I can run to produce useful debug output.