"It simply is not your overall disk IO capacity that is the performance problem with a dataset that small."
But, it clearly, and measurably is; there's nothing to argue about there. That which comes from RAM (reads) is fast, that which waits on disk (writes) is slow. Writes take several seconds to complete, thus users wait several seconds for their comments and posts to save before being able to continue reading. That sucks, and is stupid and pointless, especially since it's not even all that important that we avoid data loss. A minute of data loss averages out to close enough to zero actual data loss, since crashes are so rare.
But something still seems amiss to me.
How many new comments are you seeing per second? How many MB/s can the hardware actually write? How any MB/s of new comments are you actually getting?
Given the tiny dataset and small number of users, you really need to be running on abysmally slow (as in servers slow by late 80's standards) hardware or have a really bizarrely high user engagement or ludicrous levels of inefficiencies for this to make any sense with the numbers you've given.
Lets say an average comment is 8KB. Lets double that for indexes. At one comment per second you would be writing 86400 comments a day, replacing almost all old comments with 1.4GB of new comments in one day. Given you mention 100k comments total, presumably your actual rate of comments is much slower.
But even 16KB/second is at least an order of magnitude less than than my old 8-bit RLL harddrive for my 25 year old Amiga 500, and within reach of even the floppy drive on it...
Assuming your commenting rate is a tenth of that, the IO that should be required would in theory be within the reach of the floppy drive on a Commodore 64 home computer. Heck, it wouldn't take much for it to be within reach of the C64 tape drive.
Now, there will be some write amplification due to fsyncs etc. but nothing that could even remotely explain what you're describing unless there's something else wrong.
Maybe you have a degraded RAID array? Or lots of read/write errors on the drive?