All new toilets use the 1.6-gallon flush by federal mandate. To do that, they all flush fairly fast. It is likely that that older Eljer uses more water than the new ones, so you could not buy or install one even if you could find an old one legally.
If I had to guess, and that's all it is, the building is wet vented, which is not up to current codes. This means that your drain line also is acting as the vent(s) for the floors beneath you. If this is true, then it could be some fairly extensive reconstruction to fix since new vent lines would need to be installed for each of the floors below up above your flood plane (typically 6" above the sink or 42" above the floor), or they could exit the roof on their own.
You could probably get the same result if you were to fill the tub, washing machine, etc., and then drain all of them at the same time.