Your testing is not helpful if you admit that there are circumstances under which you have not tested. Please test under those as many people do not have the same lag you do. Also, please test using the exe available
here as it solves many common lag issues with 2.1.15's main release. Just so you know - we do not support executables you have compiled yourself, as there may be external factors making them laggier. With the exception of the primary release of 2.1.15, SRB2 executables provided via srb2.org are significantly less likely to give you anything but the best performance experience.
We know
why ZDBSP causes problems with the netcode; it's because the game doesn't check whether the length of the REJECT lump is long enough for the number of sectors in the map. The game then reads random bits of memory beyond the end of the REJECT matrix in memory as an extension, and these random bits of memory are definitely not network safe (enemies which do not see you locally may see you server side, or vicea versa). This issue is fixed in the next version of SRB2, whatever version number that may be.
The reason ZDBSP causes this problem with every map using it is because it creates a REJECT lump of the shortest possible length (an empty header lump). It IS possible to recreate the issue if you prevent ZenNode from saving the REJECT properly, but it's relatively unlikely.