There are a couple of different instances which can cause slow rendering in Camtasia Studio. To start off I would expect a rendering to normally take about the same time as the length of the timeline. For example, if the timeline is 30 minutes I would expect a ballpark rendering time of 30 minutes. Maybe a little more or a little less depending on settings.
With that said there are two things I've seen that can dramatically increase rendering time.
Using source content that has a higher dimension than 2048x2048. Since Camtasia's timeline can produce a max of 2048x2048 if you bring media into the timeline larger than that dimension Camtasia will have to scale the media down to a size that works. That scaling process will have a huge effect and perhaps double or triple the rendering time.
Adjusting the quality slider to a percentage higher than 75%. See below.
Not only does this create a much larger file it will also slow the rendering speed down considerably.