Per my automated speed tests, I too have noticed slowdowns over the Christmas period, and I'm not even with PlusNet - IDNet, using Zen's backhaul.
I think if ISPs run their infrastructure warm, squeezing every available bit of bandwidth out of it to save money, these kinds of slowdowns are to be expected. I've only noticed this issue since moving to IDNet from Zen, so in my view, they've already demonstrated they are not the fastest ISP available (even though they market themselves otherwise). On the flip side, they are a little bit cheaper than Zen, so I can tolerate this (mostly).
But yeah, a speed drop of half is bit OTT, in my opinion. Then again, it might not be, given the price you're paying?