After my iPhone 3g's (not even 3G S) last update, it became noticeably slower. It became one of the main reasons I upgraded my phone. It never spontaneously rebooted, but it definitely became slow.
I ended up getting a Samsung Galaxy S5 in 2015. I've replaced the battery once after the original started swelling, and it's still going strong. I plan to next upgrade my phone in 2018/9. I'd be pissed if the OS updated and suddenly made it very slow before then!
iPhone 3GS had a 600 mhz 32 bit processor with only 256 megabytes of RAM. The last update for it was iOS 6. Like all operating systems, iOS's memory and CPU demands have grown over time, that's likely why your perceived performance suffered.
I ended up getting a Samsung Galaxy S5 in 2015. I've replaced the battery once after the original started swelling, and it's still going strong. I plan to next upgrade my phone in 2018/9. I'd be pissed if the OS updated and suddenly made it very slow before then!