Do I go faster as Xerath if I spam Locus of Power?

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.

Some numbers:

Base Movement Speed - 315

Movement Speed with Boots - 385

Movement Speed with W buff - 493 (you lose ~2 to diminishing returns)

Speed increase from W - 493 - 385 = 108

Normal speed = 385 x 3.5 seconds = 1347 units

1.5 sec channel time + Activate W speed bonus for 2 sec bonus speed x 493 = 986 units