Apparently some change to hw/fw/sw/topology was made. If the isp's say nothing changed then you'd want to test and graph every layer2/3 / tunnel hop on the way out to the internet, including to the node of some neighbor on the same fiber or headend as you, as well as the same set of tests from an unaffected neighbor. That's a large % drop in latency, though neither that or pkt loss necessarily means anything nefarious.