Please check the below image.
The mining speed drops after "got new package" and gets back to normal after some time. Can anyone tell me if it is a normal behavior? Or is there some problem?
Probably normal. The mining software needs to feed new data to your GPUs to work on a new block. The speed drops to 0 while this is happening. It looks like this mining software is using some sort of smoothing algorithm (and you're displaying updates very frequently!), so the average over the last period of time takes a bit of time to recover until the 0-speed-period leaves the smoothing window/becomes negligible. This smoothing is just to prevent your reported hashrate (especially at 0.5s update intervals) from fluctuating wildly.