TLDR; Install Async Locator

Identifying the source

The source may be a Treasure Map in a container or something else. To determine which, check your crash report. The presence of "StructureCheck", "ExplorationMapFunction", "LootItemFunction" or "LootPool" will indicate that this is a Treasure Map issue, so please check the next section.

If your crash doesn't contain this, please create a new issue rather than replying to this.

Treasure Map containers

Treasure maps function by searching for a specific structure (such as a buried treasure, common in Shipwreck treasure chests) within a radius. This generally causes lag regardless in Vanilla, and has the potential to cause your server to crash with a "Watchdog" crash, where a tick takes too long.

As Lootr uses identical methods to generate loot to Vanilla, this isn't something that Lootr can solve.

This is a Vanilla bug.

The issue can be exacerbated by mods that increase the search radius, although I'm unaware of any mods that currently do this generally.

Recommended solution (Forge & Fabric)

The Async Locator mod can mitigate this by causing biome and structure look-up to take place off-thread, meaning that you will never experience the initial lag. Unfortunately, this mod is only available from 1.18 onwards.

Terralith-specific

If you are running Terralith specifically: This issue may happen when structures associated with biomes do not generate within the search ratio. Please see this issue on the FTB tracker for more information. There's a specific option for Terralith to fix this:

/scoreboard players set %DISABLE_MAP tr.disable_maps 1

Please note, I'm not 100% sure if this is the most recently suggested solution.

Reference issues

#100
#152
#136
#170
FTBTeam/FTB-Modpack-Issues#809
EnigmaticaModpacks/Enigmatica8#320

Ah so this is what the issue was, I legit thought I was tripping when I had this issue!

Only occurs like you said with a chest with treasure maps, and I don't necessarily get a crash but it takes AGES to stop freezing.

Mod Version: lootr-1.19-0.4.25.64.jar
OS: Windows 11
Fabric Version: 0.14.15 - fabric-api-0.73.2+1.19.2

This has occurred on a single-player setting, MANY times, I also do not crash so no logs indicating what's happening, I also do not have Terralith.

This may be available in the json, but I'm assuming there's a way to disable treasure map spawns on the loot table?
latest.log

I should probably clarify further, but there's literally nothing Lootr can do about this. It's an inherent bug in Vanilla.

@Poke097 To clarify, there's nothing I can do to help. I'd suggest seeing if someone is willing to port Async Locator to Fabric.