The cascading liquid updates happen in non ocean quarries as the quarry encounters liquids and then exposes non liquid areas for them to flow into. In the ocean the whole thing is full of water so, on each move of the quarryhead there is only one block to update - the newly exposed block. Unless you hit a tunnel but here the water falls down as its opened, ensuring that each time the quarry head passes the new blocks are already surrounded by water.
Its also common practice (a necessity) to manually flood overworld quarries with a single source block at the top, to ensure that lava encountered is dealt with. Unflooding this kind of quarry is very VERY resource intensive.
As far as I understand it, the problem is that you simply cannot just block update ONE block of water. When one block is mined it block updates the whole region of water which is what is so resource intensive.
Additionally, simply because you have a smooth fully formed ocean of source blocks on top does not mean it will be the same thing when you start chipping away blocks beneath it. What happens most of the time is actually that the pit beneath is simply filled with flowing water, not source blocks.
Now you can imagine that every time the quarries cutting head cuts away a block it causes an area to be block updated, which is filled with flowing water. Nothing except the new block might be changed, but the whole area is still recalculated. If your Quarry is running at a decent pace you can imagine the amount of calculations this totals at. Multiply that with several people on a server and again with them perhaps running multiple quarries each...
And if you REALLY need to quarry the overworld(you really shouldn't, there are so many solutions to avoid it), you most definitely should use an Ender Quarry or QuarryPlus with Pump attached. Flooding a quarry with a source block of water is one of the worst things you can do.