Same issue
I have the same issue - but only in the UK. (So far) The wifi speed at home to the ipad is 20 to 30 mbps, depending on range, so it isn't that.
it is also not the ipad, as it Farcetimed brilliantly on hols in Aus and NZ, when calling to both UK and USA.
SWMBO's given up on farcetime.
Whether the problem is Apple's, Cisco's or BT I don't know - there just is no current solution.