Another alternative could be this formula. For range 'Series Data'!Q1:Q1000000 the result was returned in ~ 1 second in my sample sheet.
=LET(_rng,WRAPROWS('Series Data'!Q1:Q120,20),
_avrg,BYROW(_rng,LAMBDA(r,AVERAGE(r))),
_stdev,BYROW(_rng,LAMBDA(r,STDEV(r))),
_res,IFNA(EXPAND(HSTACK(_avrg,_stdev),,4),""),
VSTACK("",TOCOL(_res)))