could be anything as you suspect. if i turn off all the locators, bounding boxes, trader markers (basically all of those square boxes that mark pois and such) the fps will rise above 200 for me but turn all back on, then yes it can did very low, thats why i only have the markers on that i need and not everything on.
also keep in mind that the world editor has the clouds/fog off so everything is being rendered and not hidden behing fog and such.
oh and thats testing it in a21.
in a21 the buckets are ONLY used to transport water to a spot that you designate such as you created a swim pool/spa/drinking hole or whatever.
that bucket can not be put on a campfire or any fire to boil water. sorry