So this mod has a strange impact on FPS. I spent many hours troubleshooting to come up with this conclusion:

When using this mod and no others, having a large amount of items in your inventory causes your frames to drop to very low amounts. In my testing, I used 2,000 ripped sheets (however it will do the same with bullets or any other item). This is not the same as the rip clothing stutter that others have reported, and that fixes exist for.

For these tests I used the same save file, at the same location, at the same time of day, with the same zoom level, no zombies alive, and a proper benchmark monitoring tool.

Vanilla: 240 FPS normal, 220 FPS with 2k ripped sheets in inventory.
Authentic Z by itself and no other mods: 240 FPS, 70 fps with 2k ripped sheets in inventory.
Over 200 other mods but without Authentic Z: 240 FPS, 220 fps with 2k ripped sheets in inventory.

Finally, I hosted a server for a couple friends to join with each of the 3 setups and they all reported identical results - low framerate when carrying a ton of items if Authentic Z was enabled.

I believe this is a problem in the vanilla game's optimization, but a few select mods seem to enhance that issue to be much more noticeable. Your mod is not the only one - Expanded Helicopter Events does the same thing. Kentucky Car Overhaul also does it, but to a much lesser extent.

Hope this information helps in anyway, but I'm not sure if anything can be done to fix this and it might require the game itself to be updated.

Love your mods. Cheers.