Have you read up on Time Stretch and how it works? If you’re going to be doing it a lot it’s important that you understand it.
Place individual hits on the time line or use a sampler/drum machine. Don’t stretch the loop, chop up the hits your self.
There is a setting in the preferences for the default time stretch. You can also change the time stretch in the info line, next to where it will now say MUSICAL MODE. You will get a drop down menu of the different algos. The old standard Cubase algos are there too. The elastique algos are among the best in the world.
They work by chopping the audio file into tiny pieces and inserting a gap (slower/longer) or overlapping the edges of the tiny pieces (faster/shorter), this means if the transient of your kick/snare etc is in the overlap, it will be destroyed, lost, this is what you are hearing. In the old algos you can even insert your own frequency of the splices to try and avoid, minimise the undesired results.
Other ways work more like a turntable, you won’t get splicing artefacts this way but you will go out of key as you change the size.
When I’ve made some of my bootleg mash ups, sometimes I’d use a combination of both. Using the offline processing menu (f7 i think) you can use the pitch and the time stretch windows to experiment with different combinations. But for drum loops, cutting the hits out yourself is the best result, unless you want that jungle speed up effect or slow motion sound.