Xerath's Locus of Power provides a speed buff for a short time after ending its effect, but it also takes a second or two to activate. Do you make up for the distance that you could have run while casting? Is it advantageous to spam this to cross the map quicker?
Locus of Power cannot be spammed due to its cooldown, and due to the channel time during the set up, it rarely, if ever, results in a actual increase in units/minute speed.
What you can do, in Dominion, is activate Locus of Power before clicking on the point. Since you're already rooted due to capture channeling, with this method you can get a small speed boost after capturing each point.
In general though, think of Locus of Power's speed boost as a "consolation" prize for rooting you - it shouldn't be the reason you use the ability - use it for the % Magic Penetration and range bonus.
You spend 0.5 seconds rooted for a 35% boost for 2 seconds. Assuming your unbuffed speed is R units/second, you'll move
while you could have ran normally for 2.5 seconds and have covered
So given no diminishing returns (minimal unless stacking multiple buffs) and no lag, spamming W as much as possible will make you go faster, increasing your movement by 20% of your unbuffed run speed per cast. If you spend any more than ~200 ms over the 0.5 s minimum root time, then it's a loss.