On the bog standard "Plusnet Extra" package, then management applies from 6PM to midnight, but the detailed speed page
shows browsing as running at line speed even in the evening period - and most speed testers seem to get classified as "browsing".
However, the "Plusnet Extra Fibre" package has very
different management tactics applied, that negates some of the very usefulness of fibre IMHO. The details are on the same detailed speed page
. There you get controlled on many protocols at the 5Mb and 10Mb level during daytime, *plus* restrictions in the evening peak hours - but thankfully less restricted than the bog-standard Extra package
But the one that most likely applies here is that, on "Extra Fibre", browsing is restricted to 20Mbps in the 6pm-11pm slot.
It seems like, to make the most of the fibre, you need to be streaming, downloading *and* browsing at the same time.
To be honest, if you don't need the volume of 120GB on "Extra Fibre", it could be considerably better to fall back to the "Value Fibre" package (£5 less, with 40GB limit), then add the "Pro" addon (£5 extra) to get line speed of everything, all day, for the same price.