My computer (Intel 3.1Ghz - 6G RAM) has a NVida GT620 card. When I change the settings in Blender to CUDA en GPU rendering gets really slow, (it even looks like the whole computer is slower) Changing it back to CPU and everything gets faster.
What could be the case?
To get the best performance out of your card a number of factors have to be taken into consideration. Camera resolution size and percentage, sample numbers, light paths and tile size (256 seems to be the sweet spot for CUDA). Even light bounces from the light source can play a role in render times. Be sure to have the video drivers up to date as that can affect it as well. It could also be that the card just isn’t powerful enough for Blender. Lots of stuff to look up before a proper diagnosis can be made. To start, here are 18 ways to speed up cycles renders: