There are several possible causes. Simulation of circuits that contain both reactive and nonlinear components (such as capacitors and transistors) take a lot of CPU time to simulate. You can pause and resume the simulation via the menu. Drawing of the work area (in particular, redrawing lots of rapidly updating voltage bars on pins) is also CPU intensive. You can reduce the refresh rate or turn off the voltage bars via the Settings dialog. The refresh rate of the Oscilloscope can also be reduced by right clicking on its display. Note that the next major release of KTechlab can be a lot faster in both displaying the work area and simulating reactive and nonlinear components. | 
FAQ