CPU usages of >100% (like that shown in the image above) looks like a lot in the Activity Monitor, but that's relative to 1 CPU and most computers have now have more than one CPU.
The computer in the image above has a 6-core processor. So it's effectively using 400/600% or 66.67% of the available CPU resources. If you were actively exporting with in5 at the time, this is quite good. in5 runs synchronously (in real-time) so it pulls whatever resources it can to export as quickly as possible. If you see this kind of usage while exporting with in5, it's totally normal and will drop when in5 is done exporting.
However, if InDesign was essentially sitting idle when you took this screenshot, then this is far too much (which would still be an InDesign issue...or another plugin).
You will not be able to use InDesign for other tasks while in5 is running (and in order to not slow the export, you should not be using other memory-intensive apps at the time), but you should not experience any slowdown due to in5 when you're not exporting.