[Did not affect 1.14]  as you add more and more campfires to an area the effects of lag from the smoke particles grows exponentially. Minimal particles setting doesn't change it (nor does any other setting) I usually run at 60fps, have 4GB ram dedicated to minecraft when playing. In 1.14.0 my FPS in the area where the campfires are necessary was around 38 (low but playable) but since 1.14.1 pre-release 1 and up it's been a good solid 1FPS, with the occasional spike to 2FPS. Pausing the game brings it back up to 60, so it seems to be related more to the movement of the particles than their existence in and of themselves.  Popped into a new world and tested, the lag starts to become noticeable at around 30 campfires in an area, and the game becomes unplayable around 74. (Area in question has 234 placed, yes they are necessary.) each campfire is in an uncovered 1 wide column surrounded by blocks, 4 deep (including the fire itself) - I make note of this because when testing I found that putting the same number of campfires down on a flat surface in a big block next to each other, the lag effect is not quite as bad.

Suggested solution: Particles setting to affect campfires OR undo whatever change was made to campfire's smoke-related code from 1.14.

I know a lot was done about water-logging and igniting campfires with flaming arrows, but unless the code is extremely spaghettified, that shouldn't have made a change to their smoke, somebody tried to "optimize" it, I'm sure.