I'm unsure of any hard limit in threads, but I've had people in my fellowship claim that message threads past 20 or 30 messages load very slowly on their machine so they try to restart those threads once that threshold is reached.
I personally am running a relatively recent machine (Ryzen 7 3700X, 16GB RAM) and have no desire to boot up machines that are 12 or more years old to verify this, so I'm taking their word for it.
EDIT: Dammit. I have one such machine sitting on my desk that's already booted and is used as a dedicated "document viewer/third screen" machine. I'm now very curious and it hurts.
EDIT2: Okay. My poor aging laptop (lenovo T400 - Intel Core 2 Duo [P8400], 4GB RAM) does NOT do Elvenar very well. Which is about as much as I could expect from such a machine. Aside from initial loading, I didn't notice too much of a performance drop when viewing a <10 message thread vs a 30 message thread. Overall, the experience was poor. As to be expected. F---, would use cheap $30 smartphone to play instead.
EDIT3: I used Firefox to try to play Elvenar on my aforementioned craptop. Not sure if using Chrome would've made things better, but my poor laptop would probably crumble into dust if I tried it. I did not attempt to play using Pale Moon (low resource Firefox derivative) as I wasn't confident it would run at all.
EDIT4: My curiosity is trying to murder me. I've started loading it in Pale Moon. Ran into a hardware acceleration error/warning for which I clicked "Continue to Game" and ... well. It loaded. Please send help.
EDIT5: Color me surprised. It actually works. Better in Pale Moon, actually. But only because it's not pegging my RAM. CPU's at 95% on both cores, though. Technically playable. If I had to, I could probably play the game like that.