GiffGaff run off the o2 network so effectively it is tethering the signal, therefore it will be slower than the O2 network as they prioritise their own customers and contract members before GiffGaff customers.
Why else would they call everything O2 priority? hahaha, but all in all I've had no problem with GG so far, its been decent speeds.
Hope that helps
@galfredus - assuming both tests were done on a 4G link then it illustrates the limitations of a heavily loaded gg data service.
To be fair a lot of people are switching to 4G at the moment and it is possible that gg may have to negotiate more data - but seems that these sort of speeds are fairly typical lately.
For some users 2-3MBps is acceptable but on a limited data allowance product I would hope for better. You would think that in the post-unlimited 4G goodybag world, the primary limitation ought to be the amount of data purchased, not the rate at which it can be used.
BTW I believe that gg and all other mobile networks/mvno's should be obliged to report actual data speeds delivered. I doubt they will do so voluntarily but maybe the regulators will wake up and force this to happen as it does with broadband