After testing, version 2.8 with RTX on has more realistic shadows and reflections, but the rendering speed is much lower. Maybe this version is not fully optimized?
Which resolution were you rendering the images at? When you run out of gpu memory rendering does become a lot slower, since we have to swap resources to the main memory. This has nothing to do with RTX technology in particular though. As a general rule of thumb a gpu with 6-8GB of memory is usually able to do a 4k rendering without swapping, anything above is going to be much slower.