I use macOS 10.14.1 and Matalb 2018b.
The same thing happens for me. Mine is even worse. All of the Matlab app crashes for me.
It is really annoying than just being sluggish in Matlab. I originally thought it is due to java. I uninstalled java completely but then these things still happened. Then I completely reinstalled macOS and install software. Here comes the interesting things.
With fresh macOS. Af first, everything finally works. I can use Matlab app like classification app learner etc. But later when I install other softwares, the annoying things happen again!!! So annoying!!!
Then I find this question, some app conflicts with Matlab 2018b.
I do not have magnet but some other service ran in the background. Then I end the process one by one and check whether it works. Then I find, when I kill the "popclip" process, finally everything works perfect now!