Originally posted by Nicholas Gurr, Blackpill Ph.D:

if its anything like running and jumping, you wont even notice the difference. i read somewhere that the difference between 0 running and 100 running is like 5% slower stamina depletion. a complete joke. why even have skill levels at all if they're this insignificant and you lose them on death anyway.

Well man , you read pure BS because u didnt tested yourself
100 running skill gives +25% run speed and -50% stamina usage

Looks like Its so hard to look at a wiki
https://valheim.fandom.com/wiki/Skills