It's good practice to qualify guesses about how things work so readers can understand they're (reasonable) assumptions rather than actual observations.
Any amount of magma (i.e. 1-7/7) + any amount of water (same limits), in either order results in an obsidian tile with no "leftovers" (no floor on top if a constructed stair was above when obsidianized, though). Also note that water dropped from the level above with buckets typically just evaporates rather than obsidianizing the magma, leaving the magma untouched, so the "safe" bucket brigade dropping height is 2 Z levels above. For some reason that doesn't seem to happen with water from other sources (flowing sideways, dropping from above through a pump, etc.), although it might be that the amount of water in those cases masks the evaporation cases.
Also note that there are special (odd) rules for magma flow tiles.
You can build your obsidianization casts from any material (that can be used for building, of course), as constructed walls and floors are magma safe.
Also note that since magma evaporates, you may need to time the dropping of water with the dropping of magma, or at least be prepared to make a new attempt when the previous one failed.
There is an odd feature/bug regarding revealed floors, so channeling/digging away obsidian may not result in an obsidian floor. Building a floor/road (and probably other things) on rock, at least if it's muddy, and then removing it again may result in the natural floor morphing into "soil".
While the number of items in a fortress is a factor for lag (and DF keeps track of every stone/block/etc. used in constructions) contributes to lag, you need to build a lot for that to be a significant factor. Webs in caverns will have an effect after a decade or two, and for some reason (cavern) trees seem to do so as well.