I believe it is actually known, confirmed and still not fixed bug:
0001430: Announcements vanish too quickly (next day)
https://www.bay12games.com/dwarves/mantisbt/view.php?id=1430 About impact on fps. I am on new quad cpu with 4ghz backed with 16gb ram. DF is still on old sata hd (hd system is fat32 and os is latest long term ubuntu), however I have new sata controller. I am running DF in around 45-35 fps. So impact on my fps is not currently big with 50mb gamelog.txt for each season. Clearing it still rises average fps somewhat, but it is not science any more. Many things do impact fps in overall, so it is hard to say.
This was another story on my old dual cpu with 3ghz backed with 2gb ram same (more or less) fortress run at speeds of around 10fps, which after some miracle work in 0.44 by Toady went up to 17 fps. However, I was clearing gamelog.txt religiously like every month on it. It was adding around 2 fps each time to game speed and the file was around 15mb on each purge. So it was tangible gain on my old computer.
What was your fps improvement when you cleared your Gamelog? Is this Science or just a rant?
For science you could generate 0mb, 10mb, 100mb, 1gb, 10gb and 100gb txt files and put them into DF directory and rename each to test fps to gamelog.txt. Then run DF on each and check the fps for one month. You should test different file systems. You should test ide, sata, usb and those new hds too. If there is no difference in fps on neither hd on neither file system with neither gamelog.txt, then I guess this argument will be put to rest for ever through science. It could still be an issue on some old hardware though.