The speed going round by me has always been so adulterated Ive always required at least 250mgs for an effect, and this was the decent stuff you used to get in the 90's (good powder, not 'base' paste). Over the last 10 years most of the stuff Ive come across does the trick but its so weak that half a gram is standard just for starters.
According to law enforcement agencies and forensic services, traditionally amphetamine sulphate has always been the most adulterated controlled drug on the UK black market. Im sure that that there have been other street powders on sale over the last 6 years that could easily give this statement a run for its money when it comes to low purity, but according to all of the drug literature I have read (and its a lot) UK speed historically held the title for years as britains most stepped on drug.