The most common and most likely answer to this crossword clue is the 8 letter word RAPIDITY
I've seen this crossword clue in Financial Times
R
A
P
I
D
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T
Y
More clues leading to the result RAPIDITY
Buddy explains!

The straight part
We know that "Speed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case speed = rapidity.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does RAPIDITY mean?
RAPIDITY noun- Speed, swiftness; the condition of being rapid
- A measure of velocity relative to the speed of light
- A measure of the velocity of a particle in a beam relative to the beam's axis