When using rulers that track the cursor, such as the vanishing point ruler, the update speed can drop as low as 1 or less FPS, or even freeze, when any tool that doesn't use a brush is selected. (Selection shapes, vector tools, crop tool, wand, etc) Version: 4.4.1-alpha (git 6b4d1ed)
I can confirm that this is the case for the 4.4.0 appimage on Debian 10. The rate, for me, is not quite as low as 1fps but it's notably slower than when using a painting tool. So, setting to Confirmed. I tried it with the vanishing point assistant and with the parallel ruler assistant. As for use with 'vector tools', there is no reduction in update rate if you use shape creation ('painting') tools but there is if you use shape selection of changing tools. However, they are 'painting assistants' so maybe they're only intented to be used with painting types of tools.
I looked into this. Looks like tools that are not in KisToolPaint or selection hierarchy are affected because the canvas isn't updated on mouse move.
In 5.3 this no longer seems to be an issue, either the ruler updates at correct speed with drawing tools, or it doesn't show at all (for tools in which I feel there's no need for a ruler, such as the paint bucket or eye dropper. I think it can be closed.
Marking as fixed as it still continues to not reoccur.