I think this is an issue new to 2.3.1 of GMS2 as I haven't encountered it before. This is with the newest version of both GMS2 and GMLive installed.

• Running GMLive in normal "run" (F5) mode WITHOUT server executable running: no problem.
• Running GMLive in normal "run" (F5) mode WITH server executable running: no problem.
• Running GMLive in debug mode (F6) for the same project WITH or WITHOUT server exe running: this causes a completely regular/cyclical "freeze" sequence where the game runs fine for about 1 second, followed by 1 second of being completely frozen (behaving as if it was coded to run a "while" loop for ~1000 milliseconds, for example). The debug "profiler" doesn't seem to indicate any unusual activity.

The crazy part is that it only affects debug mode, and it affects debug mode whether the server exe is running or not -- as long as the macro "live_enabled" is set to 1, just having the GMLiveAPI scripts and object in the project are enough to cause the issue, which makes me think it stems from some code run in the GMLive init scripts. Setting "live_enabled" to 0 fixes the problem entirely. 

As far as I can tell, there are no other outwardly-showing symptoms: no errors in the IDE console log, and when it is active, none from the server exe's output window. GMS2's built-in debug overlay shows nothing out of the ordinary (500 fps and acceptable levels of texture/batch swaps), nor does the debug mode's graph.

Apologies if this is a known issue or it's not widespread. Every time Game Maker has a new version, it tends to break lots of things. Aren't we all at least a little sick of that by now? :-p

If it does not show up on the profiler graph, that would be an issue with GameMaker itself rather than GMLive - I’d suggest filing a bug report for YYG with a sample project attached.

The only thing that happens once a second is a HTTP request to the server (frequency can be adjusted via first parameter in live_init), but the function is not supposed to freeze the game while doing so - let alone for a whole second.

I am getting the same exact issue, although it used to not happen at all. I was on 2.3.2 and tried updating to 2.3.3 and it didn't fix it. I'm wondering what started the problem, because debug mode was working totally fine a week ago before doing any kind of updates or anything. 

I’ve recently spotted a similar stutter on a work project when “real-time debugging” (lightning timer button on debugger toolbar) is enabled and certain expressions are entered in Watches window - simply removing watches or disabling real-time debugging would resolve this. You can check if that’s it