I would say that the aging of any mortals in the Undying Lands would not be slowed in any way.
As a matter of fact, it might be the opposite. Recall what the emissaries from the Valar said to the Númenórean king Tar-Atanamir:
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'For it is not the land of Manwë that makes its people deathless, but the Deathless that dwell therein have hallowed the land; and there you would but wither and grow weary the sooner, as moths in a light too strong and steadfast.'
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The Silmarillion Akallabêth
Though being in the Blessed Realm would have been a healing experience for Frodo especially, as a mortal he wasn't really meant to be there, so I rather think the warning of the Eldar to the Númenóreans could be taken at face value.