Treadmill running

Also: My treadmill is placed in a pretty hot room (25 degrees celsius/ almost 80 degrees fahrenheit, and I am used to running at 15 degrees celsus/59 fahrenheit.

In your case, it's either mental (boredom causing you to pay attention to heat), temperature (perhaps you're really terrible in high heat), or a miscalibrated treadmill (or perhaps the ground it's on isn't quite flat), but running on a treadmill is, in general, easier than running outside due to lack of wind resistance. Running at 15 kmph is almost equivalent to just running with a 15 kmph tailwind outside. My tempo workouts on a treadmill are consistently 8-9 seconds per km faster than they would be in good conditions on a track outside.

There's your answer. The heat and the humidity of the room.

I have to say that my experience is the opposite. I can do workouts at faster paces and for longer when on a treadmill. It just feels easier to only have to keep up with a machine driven pace. What I struggle with is the boredom of scenery not changing thus long runs can become a death march. Without that distraction, watching the digital distance meter creep upward can draw out like a blade.

What I think is so weird is that, if I were running on 15 kmph on a treadmill, I would pretty much have the same pulse as I would if I were running outside at the same pace... It just feels so much harder. However, I have suspected that my treadmill (personal owned) is miscalibrated...

I am consistently faster outdoors than I am on the treadmill.

The longest treadmill run I ever did was 9 miles at 7:20 pace. Misery! I think even 7:45 pace on a mill is a struggle.

For reference, my easy pace is roughly 7:15 and I blast through 10 milers easy several times per week.

When running on a treadmill you build up a layer of hot, humid air around your body (unless you have a fan blowing your hot air away), and the hotter body will have an increased heart rate even though treadmill running costs less energy than the same speed overground. Knowing if your TM is accurately calibrated is critical and if it is level (put a carpenter's rule on the running belt when the grade is listed as 0 to check this out). Each 1% grade increases the cost of running about 12 seconds per mile. I coached a guy who did each 20-mile long run (once each week for a number of weeks) on his treadmill, at 10MPH, which is 6:00 pace, so 2 hours once each week. He followed that with a 2:09 marathon, which beat his previous 2:12 best. Maybe just a mental thing -- if you could go 20 miles on a TM once each week, a 2:09 marathon may just fly by.

Thanks a lot!