Usualy yes,
speed depends on your infrastructure of course PC speed, internet speed, etc...
all being well ( recent ) you should have enough speed.
Things break down when the servers at
Betfairs side get overworked, then response times can get too high to be practical for speed-betting.
Remember with the Free API you cannot place multiple bets at the same time, you have to do one by one and there are limits to the number of bets per second/minuut (?) ( sorry haven't got the details at hand )
If it is part of your strategy that to green up you place bets on multiple horses at the same time then i would go with the assumption you may not have enough speed with the Free API.
Bottom line is there's only one way to really find out and that is to do a test program.
p.s.
What other answers have you found ?