You might just have a really legit treadmill. Just anecdotally, let me give you my experience. I’m a member of planet fitness and I do the same pre-lift workout all the time. 18 mins w/ first mile 9-10mph, then every 3 mins I increase by 0.4 mph from 6-18 mins. This ends up bein gun about 18 mins for 3 miles. I always do 1.5% on any treadmill.
This is really easy on the treadmills at my “home” planet fitness, and varies from one busted treadmill to another, even though they’re all the same model. I’ve even started doing it again after lifting. BUT at the other gym across town, they have those treadmills where the belt is not continuous, but more like “tiled together” - these have less bounce and unless I’m feeling great, that workout of mine is murder. I have a “hard” workout that I recently started doing, 3 miles out, 3 miles back on very flat but usually cold and windy path. With the wind 16:30, against the wind 17:30, I feel like this is much harder.
I think a lot of that might be because I listen to (good) punk rock and occasionally rap when I run on treadmills and never listen to music when I run outside. If my phone shits the bed when I’m on the treadmill or I get some shitty ad, perceived effort spikes dramatically.
Take home, not all treadmills are the same, some you can literally just bounce along, I think the belt slows over time on many. Wind really is a huge factor, there are charts calculating how many seconds, but into a steady Midwest wind is like 10 seconds for sure. Also music and distractions on a treadmill help. I think this is where the treadmill is really useful, you can basically biohack your body into overperforming. But if you only do treadmill you won’t have the same “push” strength to propel you forward and you’ll be slow and weak.