Learn how to configure correctly your graphics.
Go into options and video setting and do the following:
Perfomance = Max FPS
That is the most I can give you but you can also change others such as the ADV openGL to fast and clouds off and fog off and all those other small details.
Or it could simply be because you got a shitty computer.
And by the way tekkit is a relaly laggy modpack so it is normal you will get lags here and there!
Hope this helps! ^_^