Found a bug with snake trap. It should reduce casting speed by -10% against players (instead of -30%). Fixed, will be applied after update.
https://wowpedia.fandom.com/wiki/Snake_Trap
About global cooldown. There is two fields: UNIT_MOD_CAST_SPEED and UNIT_MOD_CAST_HASTE
UNIT_MOD_CAST_SPEED - affects spell cast speed, not displayed in character stats.
UNIT_MOD_CAST_HASTE - affects global cooldown, displayed in character stats.
Not sure, should these debuffs change the UNIT_MOD_CAST_SPEED only or both fields as it is right now. But if we find any retail videos, where we can see the character spell haste in stats after the debuff is applied and if something changes there, then it means both fields is affected and global cooldown should be affected too.