My FR10 took an age and was one of the reasons I sold it, FR610 the same. My910xt is still going strong after 4 years of near-daily abuse. Takes about 30secs to get a GPS lock.
Look out for a 2nd hand 310xt - They'll be cheap, battery lasts ages and will never die.
My Forerunner 15 is shit at getting GPS lock (>5 minutes). I've tried a factory reset, is there anything else I can try?