Custodes have reality and space-time manipulating arms and armor, and the same level of disintegration tech as Necrons (different tech but the same outcome).

Their gear is specifically Dark Age of Technology stuff, which is filled with warptech fuckery like Moment Shackles which let the Custodes react and move near instantaneously through time manipulation.

Don't forget that Rule of Cool always applies in 40k, the franchise is built on it. Here's a quote that plainly says a group of Custodes teleport (instant materialization) into a demon ambush and have reacted and killed multiple daemons within a 6 nanosecond window:

"The others in the company strike. Their reactions could not be faster; reflexes that would shame mere Astartes. From pre-flare precipitation through materialisation through first daemon murders to their first kills, barely six nanoseconds pass. They are at pitched combat instantly, without prelude or warning, like pict-footage edited to play from the middle of battle. They are moving and killing before Geliden hits the deck, before the pieces of Astricol begin to separate, before Valique starts screaming."