MC 1.17.1
BM 1.0.0
BS 1.1
Yung's API 1.17-Fabric-13
Fabric API 0.37.0

Mod list

[1.17]+Magic+Mirror+1.3.1
Adorn-2.1.1+1.17.1-fabric
advancements-enlarger-0.2.4
amecs-1.3.4+mc.1.17
appleskin-fabric-mc1.17-2.0.0
architects-palette-fabric-port-1.2
architectury-2.2.21-fabric
Artificial_Foliage_1.7_(FABRIC)_1.17-1.17.1
attachleadstologs-1.0.0-1.17.1
artisanat-2.1
autoconfig1u-3.3.1
axolotl-item-fix-1.1.1
backslot-1.1.13
backslotaddon-1.0.4
bclib-0.2.3
bedrockify-1.1.1+mc1.17
beehive-tooltips-1.3.0
beenfo-1.17.1-fabric0.36.1-1.3.2
betterbeds-1.2.0
BetterControls-1.17+-v1.2.0
better-end-0.10.5-pre
BetterMineshafts-Fabric-1.17-1.0.0
BetterStrongholds-Fabric-1.17-1.1
better-wandering-trader-1.7.0
bigbeacons-1.2.3
blockus-2.2.6+1.17.1
branders-sulfurpotassiummod-1.17-1.1.1
BuildingWands-Fabric_1.17.1-1.5_beta2
Cardinal-Components-API-3.0.1
carpet-extra-1.17.1-1.4.43
carpet-tis-addition-mc1.17.1-1.20.0+build.633
carrier-1.8.0
cavebiomes-0.6.3
charm-fabric-1.17-3.3.0
charmonium-fabric-1.17-3.3.0
clayrecipefabric-2.0.0
cleardespawn-1.17.1-fabric0.36.1-1.1.1
clientsidenoteblocks-2.0+1.17
cloth-api-2.0.54
cloth-config-5.0.34-fabric
Clumps-fabric-1.17-7.0.0.2
comforts-fabric-0.0.4-1.17
consistency_plus-1.17-0.4.0-RC1
CraftPresence-Fabric-1.17.1-Release-1.8.2
Croptopia-1.17-FABRIC-1.5.0
custom-portals-2.1.0
DarkPaintings-fabric-1.17-3.0.6
dawn-2.0.3
dazzle-2-1.17-1.0
death_controls-1.1.0
DisableCustomWorldsAdvice-1.3
dragonloot-1.0.8
DualRiders-1.17-1.0.5
Dynocaps-1.8.1
Earth2Java-1.7.1+1.17
elementalcreepers-1.4.0
EnchantmentDescriptions-fabric-1.17-3.0.12
expandedstorage-7.0.0-alpha.33+1.17
fabric-api-0.37.0+1.17
fabric-carpet-1.17.1-1.4.44+v210714
fabric-enchantments-0.3.2
fabric-language-kotlin-1.6.2+kotlin.1.5.20
fabric-searchable-containers-1.17.1-1.3.0
fabrilous-updater-2.3
fallingleaves-1.7.2+1.17
FallingTree-1.17.1-2.12.1
fast-chest-1.2+1.17
ferritecore-3.0.1-fabric
flytre_lib-1.17-0.2.21
forgottenrecipes-fabric-1.0.1
goosiks-villager-hats-mc1.17-1.2.0
grid-1.17.1-fabric0.36.1-1.6.5
grizzlybear-1.1.1
helpwanted-1.1.0
horse-stats-vanilla-4.1.8
image2map-0.2.1
indrev-1.10.0-alpha.9
inmis-2.3.1-1.17
InventoryHUD-fabric-[1.17]-3.3.2
InventorySorter-1.7.9-1.17
invisiblights-fabric-7.1+1.17
itemscroller-fabric-1.17.1-0.15.0-dev.20210707.005506
Lakeside-1.1.0+1.17
lapisreserve-1.0.8
lazydfu-0.1.2
lifts-1.2-BETA+1.17
linkedstorage-1.3.11-1.17
lovely_snails-1.0.2+1.17
malilib-fabric-1.17.1-0.10.0-dev.24
mcda-1.7.5
mcdar-1.3.1
mcdw-3.2.4
mcsa-2.3.2-1.17
megane-5.1.1
modmenu-2.0.2
morevillagers-FABRIC-1.17-2.0.3
netherite-items-1.17-1.1.0
notenoughcrashes-3.3.1+1.17-fabric
offhandoverhaul-1.1.1
onsoulfire-1.17-2
optifabric-1.11.18
Paxi-Fabric-1.17-1.2
pettable-1.2.0
PlayerPig-2.1
pling-1.4.0
PresenceFootsteps-r31-1.17-rc1
preview_OptiFine_1.17.1_HD_U_G9_pre29
replanter-fabric-1.17.1-1.3
repurposed_structures_fabric-2.1.1+1.17.1
respawnablepets-1.17-2
RoughlyEnoughItems-6.0.264-alpha-fabric
rug-1.17.1-1.1.12
scorch-1.0.2
secretrooms-1.0.0+1.17
shulkerboxtooltip-3.0.0+1.17
sihywtcamc-1.3.1+1.17.1
sihywtcamd-1.3.1+1.17.1
SimpleBackpack_Fabric-1.17.1-1.1.13
soundphysics-2.0.0
speedyhopper-1.1-1.17-1
statuseffecttimer-1.0.3
stickyhopper-2.3-1.17-1
stitched-snow-1.0.3
terrarian-slimes-1.1-BETA+1.17
tomsstorage-1.1.13-1.17.1
trident_revision-1.0.2
trinkets-3.0.2
true-secret-rooms-1.1.4
TweakMyClient-fabric-mc1.17-1.1.6+build.22
twigs-1.0.1
vanillaplusbiomes-0.3.2
veinmining-fabric-0.0.10-1.17
villagernames-1.17-2.0.0
voicechat-fabric-1.17.1-1.0.6
Walkways-1.0.1
wanderingcollector-1.0.2+mc1.17
whereisit-1.17.1-1.13.1
wirelessnetworks-1.1.2
witherite-0.0.8-hotfix
wraith-silkspawners-1.0.2
wthit-fabric-3.7.0
Xaeros_Minimap_21.13.0_Fabric_1.17.1
XaerosWorldMap_1.14.6.1_Fabric_1.17.1
xlpackets-2.0-1.17
YungsApi-1.17-Fabric-13

Problem: after entering and loading a chunk containing either a BS or BM structure, an unidentified block entity starts to take an enormous amount of time to perform a single tick, causing a severe lag.

Steps to reproduce:
for the purposes of this test, I used Carpet

1. Create a world with BM and BS installed.
2. Use /tick health to get a tick report from Carpet, almost guaranteed the average tick result is expected to be no more than 20ms (usually a lot less) and we are specifically looking for a Block Entities line. Normally, a single block entity would take less than 1ms to tick (often A LOT less than 1ms, something like 0.0xx ms).
3. Use /locate to get coordinates for any of the present structures from these mods and teleport there (or just walk/fly, makes no difference).
4. Use /tick health to get a tick report from Carpet again. The result is expected to be above 40ms, which, normally, wouldn't have been an issue, however if we take a loot at Block Entity we can see that it alone takes ~95% of that time. This causes a severe lag across the world/server, even if just 1 player encountered a chunk that includes the problematic block entity.

From my tests, I could only figure out a few things for sure: the block entity in question is created by both BM and BS; as long as no players stay within the chunk that has any of the BM or BS structures, the lag is not present; even if none of the structures a present in the Nether, it is possible to find a chunk that contains the problematic entity. At first the last find made me think that neither BM nor BS were at fault, however, removing both mods and fully regenerating the Nether fixes the issue. Just regenerating the Nether with both mods present does not. This applies to the Overworld, too.

I'm sorry as I have nothing more to give you really. The game does not crash, so there are no crash reports. Here are some screenshots I took while doing the tests:

1. While in the Nether, I discovered that as long as you are even 1 block away from the chunk with a problematic block entity, there is no lag.

Moving just a single block forward would presumably load the block entity and start causing a severe lag.

2. While in the Overworld, I discovered that the only chunks that had the problematic block entity were the ones that contained either a BS or BM structure.

This is all I can provide for now. Please tell me if there's anything else I could do.

I've been tirelessly trying to find out the cause of the lag. I dug around the areas that were lagging with an x-ray mod and a powerful drill, trying to find the cursed block entity, but after going through the entire chunk and leaving nothing behind, it did not go away. I had to assume the lag was caused by something else and generated an entirely new world. Seems to be working fine so far, even with both BS and BM installed. Being around or in said structures also seems to be causing no issues at the moment.

What I think might've happened is that I was playing on an old world generated in 1.17 without the Caves & Cliffs preview data pack, therefore the ground below my main base only lead down to level 0. And at the borders between newly generated chunks (that go down to -64) and old chunks, there was a massive void. I strongly believe that it could've interfered with the mods' generation system and caused some block entity to lag.

I will keep monitoring the situation and write back if I encounter any more issues with chunks containing structures from your mods specifically, but as of right now we may consider this issue closed as “not caused by the mod”. I apologize if I wasted any of your time.