If you had asked that a year ago, then the answer would have been yes, and there would been multiple reports of speed differences across gateways. However back in the spring/summer the gateway balancing was improved considerably, and the complaints about gateway speed or latency have pretty much vanished.
A sudden speed drop like that is much more likely to be something gone wrong locally, such as a new source of electrical interference - such as your central heating firing up now that it's getting colder, or a mains power adaptor going faulty, or a new plasma TV.
Your best bet is to run through the
Faults Checker, and let the Faults Team see if they can spot anything untoward.
