edit: Okay, I researched it. The problem is that Godot's web renderer doesn't batch shader compilation and I didn't know about it. The only good news is, the lag spikes should happen no more than once per particle effects, so it should run smoothly after 1-2min.
I'll see what I can do; perhaps I can batch it manually. Thank you for letting me know :)
Hmm, I'm also on Firefox.
Does it lag for you in the early stages or "only" later on? If it's early on, it might have to do with WebGPU and I'll have to look into it.
If it lags by the time you get further up, that's more expected. I'm playing on a ~6 year old laptop and all my friends played it to completions, but especially the damage and money labels sap a lot of performance atm.