Exceptionally slow start building floor (9)

I believe the answer is:

dumbfound

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'floor' is the definition.
(thesaurus)

'exceptionally slow start' is the wordplay.
'exceptionally slow' becomes 'dumb' (synonyms. I am not sure about the 'exceptionally' bit.).
'start' becomes 'found' (I've seen this before).
'dumb'+'found'='DUMBFOUND'

'building' is the link.

Can you help me to learn more?

(Other definitions for dumbfound that I've seen before include "are astonishing" , "overcome with astonishment" , "Astonish, render speechless" , "Leave lost for words" , "are astounding" .)

I've seen this clue in The Sun.

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