What is tyre tension? Do you mean tyre compression? i.e. clamping pressure at the roller? That will make a difference to the power required to reach a given wheel rpm. If you're using resistance curves to estimate power based on roller speed, then this is important to keep consistent but if you are using a power meter or basing effort on hear rate then it is irrelevant.
Since you mention comparison of indoor and outdoor effort difficulties when way below threshold so I expect you have a power meter and you're using power in both cases? If so, the thread title has nothing to do with the actual issue.
This would IMO narrow the difference you're experiencing down to one or more of a small number of factors.
- Ventilation and heat. Do you seem to sweat a lot indoors compared to outdoors? It's not just that the temperature may be different indoors, it's also the lack of ventilation. Without airflow, heat transfer including that due to evaporation of sweat is massively reduced. This can make a huge difference to your ability to produce power and your perception of the effort.
- Psychology - Stimulation, motivation and distraction. If you sit alone in a room on your bike and simply stare at a number and try to maintain it, that makes it considerably more difficult, especially for prolonged periods, than getting outside and experiencing external physical and visual feedback from your efforts and your environment. I find using Zwift definitely helps massively but it's still nowhere near as good as getting outside. I'm sure my best efforts have all been out on the roads, though I am more consistent on the trainer with sufficient cooling and a plan.
- Different mechanics/feel of static bike versus the road. Pedal resistance from a fluid trainer feels much better than magnetic resistance IMO and it's better than the electronic feedback I've tried in my very limited experience with these, however it still feels different from what you feel on the road. Size of flywheel, rpm/resistance properties, your cadence, your pedalling technique and power levels all effect the similarities or otherwise of pedaling on a static trainer compared to the road. Going up a steep hill with low cadence and low speed is very different than pedaling at low cadence but high speed on the flat using the same power. This undoubtedly impacts your perception of difficulty and your muscle usage and fatigue. Furthermore, your body probably moves quite differently relative to the bike when it's locked in place. I know I genreally find the saddle less comfortable on a trainer. This alone used to be enough to limit the length of my trainer sessions but I seem to have gotten used to it!