I would have thought deliberate throttling would come without an increase in the ping time.
Try the TBB 1GB download test file using a download manager (something like FreeDownloadManager
Using a 12 split (i.e. splitting the file into 12 sections and downloading simultaneously) I'm getting a download speed of around 4558kB/s (around 36.5Mbps) - i.e. practically full speed.
If I download the same file using a single thread (e.g. downloading with IE) I'm getting a very bouncy download of between 6-14Mbps.
In my past experience (and I've been with some of the worst ISPs for congestion - Eclipse, Plusnet, ACE, Entanet) congestion has normally affected total available speed no matter how many segments/sections/threads are involved in the download.
I.E. A multi-thread Usenet download would also slow down when congestion is present.
Apart from deliberate throttling (although I'm sure someone will come along and correct me) I don't know of many issues which would cause a single thread to download significantly slower than a multi-thread.
I think we can eliminate PC and setup problems (as the issue corrects itself after peak hours have passed) - if you had MTU / Rwin / wireless / etc. issues surely the speeds would be low at all times (you wouldn't get 37Mbps downloads at 2am and 4Mbps downloads at 8pm if you had your MTU frame size set incorrectly).
It's possible any throttling (and it remains to be proven - either way) could be in place to prevent network overload caused by a sudden switch from 40/10 to 80/20 products.
It wouldn't be the first time an ISP has throttled some users, at peak times, to protect their network. Plusnet did (and most likely still do) this and Entanet had a very elaborate "throttling" system in place which kicked it (at varying levels of intensity) as their backbones started to overload.
Maybe the happy time is over and we're starting to wake up to the impossibility of providing a "real" unlimited product (at such high speeds) for £30 a month?
vDSL2 FTTC Infinity with BT
DL Sync 40Mbps
UL Sync 10Mbps