data0={<|"a"->1,"b"->4|>,<|"a"->2,"b"->3|>};

Clear[data];
data=data0;

RepeatedTiming[Do[If[OddQ[data[[i,"a"]]],data[[i,"b"]]=2*data[[i,"b"]]],{i,1,Length[data]}],30]
data

Clear[data];
data={<|"a"->1,"b"->4|>,<|"a"->2,"b"->3|>}

RepeatedTiming[Table[data[[i,"b"]]=2*data[[i,"b"]];,{i,Length[data]}],30]

Clear[data];
data=data0;

RepeatedTiming[data[All,If[OddQ[Key["a"][#]],<|"a"->#["a"],"b"->#["b"]*2|>,#]&],30]

data