It looks as though this one radiator is piped the wrong way round, but that would only cause a flow problem with old-fashioned one-way TRV's. Modern ones accept flow in either direction and have flow arrows pointing in both directions. If yours is a uni-directional one, just change it for the new type, as it really doesn't matter if the hot enters via the IRV.
I agree with DONFRAMAC but the most likely reason the radiator is slow to warm is because hot water is finding an easier route round the system than through that rad. I would suggest balancing the system first to see if this solves the problem. Read the article on how to balance central heating in the projects section of this website.
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