Long loading times and lag are a sign that your device might probably struggle with the game's requirements. It's a 3d game after all, with lots of resources, and it does come with a set of higher requirements. You can turn off or lower some of the options (like sweat and dirt) and use FSR to increase performance and reduce lag (long loading times will likely remain).
When it comes to unfairness in battles, the CPU does consume stamina, but also makes sure to completely decrease their effort on occasion in order to restore enough stamina to not run out. Have you tried periodically decreasing your effort all the way to zero, in order to restore your stamina when it was running low? You cannot always increase effort and not expect to run out of stamina. The game is actually centered around the idea of effort management - otherwise all you'd have to do would be to swipe up from start to end and win. Effort management is not complex: if you cannot win an action, investing effort and stamina is pointless, so it's better to cut your effort down to 0 and restore stamina. You need to do this periodically, if you want to win.
Also, under Options there are various difficulty levels, that make the game easier or harder.
Both the player's and the CPU's stamina also restore over time if not used, and except for the hardest difficulty level, the CPU's stamina actually restores slower than the player's.
Hope this helps, if you decide to give the game another try. I'll take your feedback on board regardless, since I realize I probably need to do a better job of providing clear instructions