Speed in Chinese / Japanese...

高速 means “high speed” or “in high gear” in Chinese and Japanese Kanji.

In old Korean Hanja, it also means “high speed” but can also be a nickname for “rapid transit.”

高速 is the best selection if you want to say “speed” in regard to your race car, race boat, or lifestyle. You need this word, which literally means “high speed,” as the Asian word for “speed” alone does not suggest whether you mean fast or slow.

緩急 is often used as a Korean martial arts term, “speed control.” It's also one of the 8 key concepts of Tang Soo Do.

In other contexts, this can mean priority, pace, tempo, or slow and fast.

快 is the single character for “fast” in Chinese.

This applies to race cars, airplanes, rockets, etc.

This can also mean rapid, quick, speed, rate, soon, to make haste, clever, or sharp (of knives or wits). In some context it can mean cheerful or happy.

This is not a common choice for a wall scroll but if you're a speed freak, this may work for you.

Note: In Japanese, this usually has a meaning of pleasure, cheerful, delight, enjoyment, or the female given name Yoshi.