Best way to knit a scarf fast is to use a fluffy wool and very large needles.
One I have used a lot is a free pattern for a scarf using Fusion wool.
I haven't got the pattern with me but it is the same for every row so I think I can remember
Size 12 needles (UK)
cast on 18 stitches
Knit 2, yarn forward round needle, purl 2 together, repeat to last 2 stitches, knit 2.
I'm not sure if that is right I will check the pattern when I get home and edit this post tomorrow.
You don't have to use Fusion wool but it is gorgeous with complementary colours shading into each other. One 50g ball makes a short scarf or 2 balls for a long scarf. It takes me about 3 and half hours to knit a long one.
I have checked the pattern and it is correct. If you are not doing a pattern the scarf grows faster with stocking stitch, alternate rows knit and purl, rather than just knit throughout.