Questions about debugging a particular area of lag

Bit of an open question, I'll try and be concise.

On a 1.7.10 server modpack, all players experience significant lag when logging in to the server and when respawning. I have a hunch that it's waiting on some mod for player-related data to save or load causing it. Questions are:

1) Is this actually possible?

2) Could somebody summarize the "spawn" logic for me or point me in the right direction to examine the flow of code for login/respawn of players? I can check this out easily in an Forge-src workspace.

3) Does anybody have any tips on where I might be able to throw in some debug info in source to diagnose this? (I've never compiled Forge from source before, I'm hoping it's not too convoluted a process).

Or, better yet, is there some kind of Java tool that I can use to debug or... generate a stacktrace whenever a method is called...? Or something like that? I guess similar to a breakpoint in an IDE but to do with compiled code (since I obviously can't run a modpack server with forge sources)? Or if any more experienced Java developers could point me to any other tool that might assist me in diagnosing it would be great.. if such a tool like that exists it'd be pretty awesome.

Thanks!