The most common and most likely answer to this crossword clue is the 8 letter word TERAFLOP
I've seen this crossword clue in The Guardian
T
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More clues leading to the result TERAFLOP
Buddy explains!

The straight part
We know that "Calculating at this rate, say" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case calculating at this rate, say = teraflop.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 TERAFLOP mean?
TERAFLOP noun- A unit of computing power equal to one trillion (10¹²) floating-point operations per second.