1]When you play a DVD for the first time kaffeine has to decrypt it with libdvdcss. This sometimes takes a few minutes. Kaffeine / libdvdcss generally "remembers" the encryption, so next time you play the DVD, it'll start faster.
2]When watching video with kaffeine, you can turn deinterlacing on and off by pressing the "I" key on your keyboard. Choose whichever setting looks best for you.
Deinterlacing options are a trade-off. If you have a fast CPU, you can select "Very very high CPU usage" and you will get a good picture. If your CPU is too slow, you may find that this option means that kaffeine will drop frames as it cannot de-interlace in real time. This will give you jerky playback.
So, there's no "correct answer" - try the different options and find which works best for you, your display, your video, and your CPU.