Need help with performance issue

In the game I DM (my Lost Mine of Phandelver campaign), I have no performance problems at all.  My players are on a large map I created for the Wyvern Tor caves, which I created using the "Save vs Cave" tile sets.  We use dynamic lighting extensively.  All my NPC tokens are set to not emit any light and don't have "Has sight" checked, I manage all of that via the measurement tool.

In the game I'm a player in (The One True Campaign), we had horrible lag across all systems and I'm trying to understand what my DM can do to fix this.  We are playing the Storm King's Thunder campaign, but we were on a custom map he had built.  I don't know what tile set he was using, but he also had dynamic lighting enabled (set to update on drop only).

I did some comparisons yesterday, using Chrome (65.0.3325.181, 64-bit) and Firefox (59.0.2, 64-bit).  In both browsers, my map idles at less than 0.5% CPU usage on my Core i7-6700K.  It spikes up to around 3-4% when I drag tokens around, but quickly settles back down to basically zero.  I tried adding 20 Zombies to my map to mirror the large battle we had in the other game, and nothing really changed.  However, in both browsers, the STK game idles are more like 8% active CPU, and spikes as high as 20% when I drag tokens around.  Performance was significantly worse on the laptops we were using during the actual game session.

My DM is going to check the sight/lighting settings on the tokens he was using, but it feels like it's more than just that.  Any other suggestions of things we can look at?

Most common factor causing this, that I've seen: The number of Characters (times the number of attributes on each character... a heavy-weight character sheet + a lot of Character Journals can add lag. If your DM has done something like, import a bunch of monsters as Characters, that can cause lag.

If the game was made with SKT (as you mentioned) that is already one of the largest single-game products in Roll20 Marketplace (it has a lot of Characters and tonnnns of Handouts in the Journal), this reason alone might be the difference of the 2% to 8% resource use.

More places to check/consider that could affect processor cycles,
Dynamic Light -- whether it's on or off is a difference, but also on the "Custom Map" you mentioned, if the DM made very complicated DL walls, or tons of light sources, that can cause lag.
Advanced Fog of War settings.
Special FX (particle effects). If you have a bunch of those going (PageFX plugin, or by giving FX access to players via macros).

Clear browser cache. (Probably won't solve it in this case but doesn't hurt.)
Alternate Chrome to Firefox.
Take a look at (consider toggling) "Hardware Acceleration" or other settings relating to WebGL in your Browser and OS.
My personal experience in the last 6 months has been problems with WebGL & Hardware Acceleration in Chrome, so I switched to Firefox and it currently works better for me when using Dynamic Light.

Great, thanks for the tips.  For reference, I was driving a TV from my laptop via Firefox so everyone at the table could see the map, and it was still pretty laggy for me (though worse for the DM and the other player at the table who had a laptop and was running it locally).

Edit: I just fired up my Princes of the Apocalypse campaign as a test, and even running both accounts (GM in Chrome, observer in Firefox) on two separate monitors on the same computer, both browsers are idling at less than 1% CPU usage.  I haven't seen STK on Roll20 but Princes seems fairly large as well, and that seems to run just fine for me.

Edit 2: Another thing that seems strange is that the STK custom map is idling at 25% GPU usage, according to Task Manager.  My LMoP custom map idles at 0% GPU usage.

Not uncommon for GM to use more processor than Player view, since GM has addition of GM Layer with its transparency, Plus Opacity seeing through any Dynamic Light or Fog of War, plus seeing Light sources including any-all even ones that are not visible to players. Plus background loading-caching of any-all characters, handouts including those that are not enabled for Player permissions.

I've heard that SKT remains the largest, not sure by what margin. The Aaron can quantify the contents of both Module products, if he sees this thread.

Tell more about the Dynamic Lighting set-up on the Custom Map page that is on 25%? Maybe choking on too many squares (example: 75x75 with .5 grid size fraction), too many light sources, or too many wall segments, those would be some areas to consider trimming down on a custom page that's spinning the processors.  Or is it a hex grid? Consider the Page dimensions, DL settings, Fog settings, number tokens, number light sources, and shape/complexity of DL walls.

To be clear, I'm the GM of the game that has idle at 0%, and a player of the one that idles at 8% or more GPU and 25% GPU (so one could assume that my GM has even worse performance in his game than I do).

The custom map was quite small, I will find out the exact dimensions but I'd be shocked if it was more than 50x50 and probably more like 30x30 with a 1.0 grid size fraction.  No light source objects that I'm aware of, the NPCs probably has "has sight" checked but they didn't seem to be emitting light.  The tile set walls were all straight with 90 degree corners, and the DL walls appeared to match this.

I'm going to ask if he'll flag me as a GM so I can poke around the map a little more and run some more experiments to try and figure this out.

Okay, my GM flagged me as a GM temporarily so I could poke around.  Map size was 38x25, which seems pretty reasonable (I've certainly made much larger maps without issue)  There were a few light sources, but deleting them made no difference.  There were a few NPCs that had "has sight" checked, unchecking that made no difference.  The DL walls were all straight and seemed very reasonable, based on my experience setting them up myself for other maps.

Turning off Advanced Fog of War seems to have done the trick, however.  Idle CPU dropped from 8-10 down to 0 immediately, and GPU usage also went from 25% to 0.

Here is a thread for reporting issues or troubleshooting possible bugs with Advanced Fog Of War, a fairly new feature added on Roll20, 
https://app.roll20.net/forum/post/5576650/advanced...

Cheers, I'll post my info in that thread.