"certainly slow" is a correct phrase in written English.
It can be used to express that something is slower than usual, for example: The process of achieving the desired outcome was certainly slow, requiring months of careful planning.
Exact(30)
"I think that could certainly slow them down a bit".
The fall in commodity prices will certainly slow things down.
But it will certainly slow corporate restructuring across Europe, particularly in Germany.
America's housing bubble might then burst and consumer spending would certainly slow sharply.
It did, based on our statistics, certainly slow down the rate of construction.
Sulphate-based geoengineering would certainly slow down its recovery, but would not send it into reverse.
Similar(30)
The world's economies are certainly slowing fast.
"The speed of the downturn has certainly slowed in the last few months," Mr. Dales said.
Theatre beyond Edinburgh has not quite stopped, but it has certainly slowed down.
"The number of infections has certainly slowed in Iowa," Olson says.
It's certainly slowing in the United States faster than, I guess, the industry would have predicted several quarters ago.
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com