The slowness is meant to represent the distance they are traveling, in both time and space. This was also made in 1968, the moon landing was in 1969. Compare Planet of the Apes to 2001 for a good comparison of what special effects were like in the same year.
Most of them are long winded, it was the style of the time.
If you think of the movie as 3 parts. One, pre-man discovers tools (because the monolith changes one tribe). Two, Man must overcome the tools it has created. Three, man is absorbed by the aliens tool to become next-man.
Anyways, I understand why someone might not like it but it is one of my all time favorite movies and its worth watching later in your life as you might get different impressions on it if you are young now