We ve been in Huay Xai in August for our first time and didn't decided upfront how we get from there to Luang Prabang. We were very irritated about the costs and time for travelling.
There are 2 options to get to Luang Prabang:
1. with the Slowboat in 2days to Luang Prabang with a separat stay in Pak Beng for 170.000kip p.P. plus overnight stay. People told us different stories about travelling with the Slowboat depending in the travel season. In the wet season the boats where very empty and you could sit. In the dry season it should be quiet different because then the boat is packed with more people then seats. So 2 days can be very hard and long. The luxus boats take the same time but cost more than 100$ p.P. (or even 400$)
The 2. option was going by speedboat: 1500THB p.P. in 5-6hours to Luang Prabang. We thought we give it a chance even though the internet recommended not to do so.
As we arrived at the peer 4 Travellers were waiting with us. The boat was very small and the boat was packed full with local people and their luggage. There were no seat or anything to sit on. We had to sit on the ground (space: 70x70cm). We made sure that our backpack was securly fasten and started the trip. So far everything went well but after a while the seating position became very uncomfortable because of the small space. After a short stopp in Pak Beng (after 2,5h) we went further on. The biggest problem that came then was the rain. Because the boat was going quiet fast and the rain drops hit the face like small needles. Even rain jacket and Rain poncho didn't help at all. So we drove 3h in the rain and we got totally wet.
The wet cloths and the wind made everthing very cold but we where happy when we reached Luang Prabang.
For sure the Trip was a real adventure and we might not do it again but if you don't want to travel in an maybe overgrowded boat or want to pay for a luxus version more then 100$ the speedboat was the Option for us but sure not for everyone. You should have faith in the driver and be careful and still then nothing is 100% safe.