The main point to notice is, when the bikes enter the slow section, the time between the two bikes remains the same but, only the distance between them shrinks down. This shows that even though 2 seconds in the fast section looks like a bigger gain, it is just a false sense of gain. When the riders enter a slower section that big distance gain is reduced to a lot smaller distance. The opposite holds true also. The distance gained, by 2 seconds in a slow section, will stretch out when entering a fast section but, the time difference will remain the same.
When working on improving your lap times work on the section were you have the most room to shave off time. If the slow section is your weak point then work on that first. If the fast section is your weak point then work on that. It doesn't matter where you gain the 2 seconds, fast section or slow, you will still get back to the finish line 2 seconds sooner, wherever you gain it.