Slow has nothing to do with frost resist, it's not an inherent property of cold, but a separate attribute entirely (though it is tied to Ice Shards, and maybe a few other cold items - Frostbite shrines have slow, and there's a rare prefix that has cold damage and slow). There is no resistance to it, afaik - I've used it on every boss in the game
10-20 fire damage, average 15 fire, +10%, 16.5 fire damage +6 ice damage, 16.5 fire damage, 6 ice damage, 21.5 total damage ignoring resists
Though that specific modifier you mentioned is fire/frost/shocking, a rare suffix that adds both a +% element, and a flat amount of that damage type. As far as I know, flat amounts are added up BEFORE percentage modifiers are applied (to give you the highest possible damage, eg, 20+20%+2 would give 24, but 20+2+20% would give 24.4)
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