Replacement VBA for sumif on a large set of data - taking too long to calcuate

Hi all,

I have a VBA code that runs a lot of formulas and variables over a large set of data (over 50k rows). When I have gone to run the formula it is taking around 10minutes to complete each time, through process of elimination (commenting out segments of coding bits at a time) I have deduced that it is the following code that is slowing it down which uses sumif formulas.

Range("T2").Select
    ActiveCell.FormulaR1C1 = "=SUMIF(C18,RC19,C11)"
    Range("T2:T" & Range("AD3").Value).FillDown

    Range("X2").Select
    ActiveCell.FormulaR1C1 = "=SUMIF(C22,RC23,C11)"
    Range("X2:X" & Range("AD4").Value).FillDown

    Range("AB2").Select
    ActiveCell.FormulaR1C1 = "=SUMIF(C26,RC27,C11)"
   Range("AB2:AB" & Range("AD5").Value).FillDown

I have searched through the forum and have found some helpful threads which replace the sumif formulas with different coding (scripting dictionaries etc) but, my VBA knowledge is intermediate at best and I am struggling to understand the coding, I wondered if anyone could help me implement this into my code to speed the VBA and whilst explaining the coding at the same time so I know what it is doing for future reference?

TIA