The most common and most likely answer to this crossword clue is the 5 plus 3 plus 6 letter word WORDS PER MINUTE
I've seen this crossword clue in The Sunday Telegraph
W
O
R
D
S
P
E
R
M
I
N
U
T
E
Buddy explains!
The straight part
We know that "that’s used to evaluate speed of performer in pool?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that’s used to evaluate speed of performer in pool? = words per minute.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.