My two cents worth: neither my wife Ann nor I have ever had altitude sickness but have many friends who complain about it.
Most of our trips to, say, Summit County, CO to stay at the Tiger Run trailer park in Breck from Ohio are by RV, it takes us two and a half days of traveling, gradually gaining altitude, which may help in not getting sick. We usually stay for three weeks but see no difference in breathing from the first to the last day, but there may be.
Last year we flew to Reno and stayed in South Lake Tahoe, I don't know the altitude there, but it is lower than Breckenridge. We also had no problems there. No problem this year in Lech, Austria, though the mountains seem much higher than in the rockies, in fact the valleys are lower and many mountain tops are not as high as the town of Breckenridge.
BTW, Going from Breckenridge to Frisco you lose about a hundred feet, if that.