@Flupster I only have KSP 1.5.1 so I don't know if the official build of this mod causes this problem, but after a few hours of uninstalling all mods then about 20 iterations of reinstalling a few, reloading, and testing, I have finally determined that your build v5.18.1 in combination of Kerbalism v2.0.0.0 causes massive stutter/hitching every few seconds while Kerbalism is transmitting science data back to KSC. No expectation for you to fix this, but thought you might be interested.
edit: Did a little poking around in the source. in xScienceEventHandler it does a refresh every second whenever the players science amount changes or science is sent to KSC. In Kerbalism, the data is transmitted constantly, so those events both fire constantly but just far enough apart so [x] is refreshing just about every second or two the entire time (could be minutes long or longer). The faster the data transmission speed (so closer to KSC), the more pronounced it is. Changing lines 257 and 263 to use (false, 5) instead of () stops the stutter entirely during transmission, as it delays the refresh until it's all done. There's still a single stutter when [x] finally refreshes, but that's normal behavior for the mod anyhow.