Tl: DR.
but your issue is not how much speed you are adding, your issue is how OFTEN you are adding.
Create a variable that adds up the totals of your speed*delta
create a counter that ++ on tick.
branch when the counter >30 (30 frames passed)
Print out a string with the total of the speed. Reset the counter.
If the 2 results match you can assume the same speed is being added to the input.
on a system with 60fps you add 60 times per second.
on a system with 30fps you ads 30 times per second.
Instead of equalizing via a different speed.
Add the same speed. Change how often you add it so both systems are forced to match.
Add the tick amount every frame. As soon as the added value is greater then .999 reset it to 0 and add your movement input node with your preset speed.