Extreme speed and rapid speed both referred to the concept of speed, but they were different in degree. Extreme speed referred to the highest speed or extremely fast speed, reaching the limit of speed. And rapid speed referred to extremely fast speed, usually used in emergency situations. In terms of speed, Extreme Speed was faster than Rapid Speed. Fast speed also referred to extremely fast speed, which was similar to rapid speed. In summary, extreme speed was the fastest, and the speed of extreme speed and extreme speed was relatively faster.
The SCP Foundation is purely fictional. It's a collection of imaginative and often creepy stories and characters that are made up for entertainment purposes and have no basis in reality.
Crossover fanfics also attract new fans. If someone is a fan of another franchise and then discovers an interesting SCP Foundation crossover, they might become interested in the SCP Foundation itself. This helps to grow the community. For instance, a 'Star Wars' and SCP crossover could draw in 'Star Wars' fans who are curious about how the two would mix.