Rate of reaction of silver salts of carboxylic acids having different alkyl groups towards Hunsdiecker Reaction Hunsdiecker reaction follows free radical mechanism but it is given that rate of reaction with different alkyl groups (R) attached to silver salts of carboxylic acid follows the order as follows

Primary > Secondary  > Tertiary  (Rate)

But free radical stability is exactly opposite!

Primary &lt; Secondary &lt; Tertiary  (Stability Free Radicals)

What can be the reason for this kind of trend in this reaction?

I'm attaching the reaction mechanism for reference.
If the free radical is less stable, it will react faster. 
So once the system is started, the propagation (step 4) will proceed faster for primary radicals. 
That means that it is the limiting step and that step 3 is faster than 4 for any type of R.