Why does the context menu take forever to open in Kdevelop?

I'm running Kdevelop 4.6.0 on Linux Mint 17.3, Mate. I've tried other version of Kdevelop but they have the same issue. When you right click in Kdevelop to open a context menu, I can *literally* count slowly to 12 before the context menu opens. Every single time you open the context menu. Pressing the context menu key on the keyboard has the same effect. How can I find the problem and fix it?