There are no prizes for high-speed hiking, but there are many rewards for those who go slow enough to appreciate the scenery and observe the wildlife and wildflowers. Take a good rest and see if there is any improvement. If there is none, you should head back and spend more time acclimating. Often a camp midway up the trail will help you acclimate and it will break the hike into more manageable segments. Weather Weather is another major factor.
High mountains tend to foster unpredictable weather.
It is not unusual to see snow or sleet on the peaks in July. Nights are cold and, depending on the altitude, often below freezing. Afternoon thundershowers accompanied by wind are common during the summer. Be prepared for changing weather. Carry several layers of clothing, including good rain gear, on all hikes.