Hey, I am running an Origins server, and we are having an issue with both the Celestium and Aquarium armour full speed boosts plus farmers respite coffee spped boost plus origin speed boos plus potion of speed boost making players far too fast.

If there was a way in the configs to manually lower the full armour speed boost, it would be much appreciated - especially the aquarium one as it is incredibly easy to get for water origins who already get speed boosts in water, just their origin speed plus the armour is starting to cause chunk loading issues.

This is actually a really interesting issue. I have investigated this in the context of Mythic Metals + Origins, and have made a discovery.

Normally your movement speed is completely independent from swimming speed. You can test this yourself by going in-game, giving yourself Speed 20 via /effect, and you will note that your swim speed hasn't changed. The discovery is the fact that movement speed affects your swim speed when wearing Depth Strider boots. Putting on a pair of leather boots with the enchant, and giving yourself Speed 20, verifies this theory. I am uncertain on whether this behavior is intentional or not, as it is very plausible to be a bug in vanilla.

As you have noted, that when people are using Aquarium Armor they are getting an incredible speed boost - This isn't exactly the case, as the bug is triggered specifically with the boots, since they give you Natural Depth Strider. This acts the same as the enchantment, and you can also test this side-by-side with a pair of any enchanted vanilla boots.

This is something I will note down for future balancing, as I am working on an Aquarium rework which does focus on granting the player swim speed. Turns out Celestium is just faster regardless due to this "feature"...
I also believe that lowering the speed boost doesn't fully solve your issue. In a context of Origins: with any pair of regular Depth Strider boots, a Dolphin, and a Merling origin you can still achieve incredible server-crunching speeds (~118 blocks per second in water with Speed 2, thats fast!)

Long-term I am planning on making attributes of things configurable. It's not a high priority however, as feedback like this is super-valuable in balancing. When Mythic Metals goes out of beta (which is not too far off in the future, sometime next year hopefully) I do want to make most of the aspects of the mod configurable.

Overall I hope this proves to be insightful in how to handle this problem. I think the only real fix would be finding some mod that caps swim speeds, as I know from experience this particular issue is fairly easy to re-produce in a lot of modded contexts.