c# - XNA's GameTime.ElapsedGameTime.TotalSeconds isn't working as expected -

as i've seen @ quite few website, best way handle variable frame rate , movement (moving projectiles, monsters, swinging swords)... looks this:

position += totalspeed * (float)gt.elapsedgametime.totalseconds; 

and in game's main constructor (game1.cs):

        isfixedtimestep = false;         targetelapsedtime = timespan.fromseconds(16.666); //16.666 milliseconds, or 60 fps.          graphics.synchronizewithverticalretrace = true; 

i started notice whenever fps @ 30, projectiles (in case, arrows) move slower when @ full speed. turning v-sync false above unlocked frame rate i'm going around 120 fps. projectiles going faster.

unfortunately, applies else.

am doing wrong, or totalseconds unreliable/broken? plan switch on monogame eventually, i'm not sure if fix issue.

thanks in advance.