Too dense to keep up with the times (4)

I believe the answer is:

slow

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'too dense' is the definition.
'slow' can be an answer for 'dense' (synonyms). I'm unsure of the 'too' bit.

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for slow that I've seen before include "Not quick to understand" , "Lento (mus.)" , "Intellectually dull" , "Lacking in pace or interest" , "Unhurried" .)

I've seen this clue in the Evening Standard.

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