Boots of Swiftness grant the most base movement speed across all boots, excluding Boots of Mobility whose effect is only temporary. Because of this, these boots are also the most effective with percentage movement speed increasing effects.
The defense mastery Swiftness stacks multiplicatively with Boots of Swiftness to provide a total of 32.5% reduction in the potency of slows.
Sejuani's passive also stacks with Boots of Swiftness to provide a scaling reduction in the potency of slows from 32.5% (at level one) to 43.75% (at level 17).
You can reduce the potency of slows even more by having the Swiftness defense mastery. Having both of these on Sejuani makes the Slow Potency Reduction scale from 39.25% (At level one) to 49.375% (at level 17).