You can use two items that grant a non cooldown skill, like seal of skies, and alternate your casts between them to get higher casting speed. This boost to casting speed varies between skills. Recorded on playtest version, this character has 175% casting speed (195% in biting blades example).
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This is an old feature 
Iirc I discovered and reported this feature during the 1.2 public playtest in 2023, but the playtest forum has been wiped since so I can’t find that report. I still have some builds using it, like this one Spellbreaker, Level 100 (GD 1.3.0.0) - Grim Dawn Build Calculator Looks pretty wack, but was able to reliably clear old SR65 at least 
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