This is an interesting topic that I was also thinking about recently. I am in Tonga and have been experimenting with the different sellers at the market, buying a kilo bag each Saturday morning. I had one very finely ground batch, and during prep lost about 40% of the mass whilst massaging and straining. I collected most of this prior to drinking, with 4 filters through the stocking and agree this is partly wasted kava as it has only been soaked and not massaged/squeezed before disposal. It was strong and had good effects but I was noticeably slower than normal the mornings after drinking this batch.
Switching to a more medium grind supplier the last few weeks and I get much less sediment, and less lethargy the next morning.
On the subject of nausea though I think Henry is on to something - nausea is common in drinking fresh kava also, which has no sediment. Some of the nakamals in Vila do not do a particularly good job of cleaning their kava during afternoon prep, and leaving too much skin on gets people retching in the dark. Even good, clean, but very strong fresh kava can get people vomiting of they take too much too quick. So maybe the nausea effects are more to do with multiple properties of kava hitting your stomach.
The worst ever nausea that I have had is when fresh (green) kava is prepared and left in the bucket more than 3-4 hours and it "turns", like in some of the Vila nakamals that open mid afternoon, so prepare their kava earlier around lunchtime. By 6 in the evening it has a terrible sour, acidic, face-gurning property to it. You feel the nausea slam your stomach lining immediately after downing the shell and have to really focus to hold it. I don't know what causes this "turn" (oxidation?) but would be interested to find out.