Do we as players have speed priority over cpu trainers

self explanatory title, why does the controllable character always attack first in game, except a very few instances?

uhhh.. no we don't always attack first haha. never used a Torterra? :p
a lot of the time we tend to be a little higher level than AI opponents - that's just how the level curve works. this will of course lead to us having higher speed most of the time, unless we're facing an opponent with a particularly high base speed stat.

As in like in speed ties/ instance when you and the opponent have the same speed? I don't think the game favours you, I think it's just that it's random.

I tried searching the wikis, and found nothing.
Yes all that is true, but remember we always attack first than the gengars and similar pokemon at the elite four even when underleveled, also when raising the level of a pokemon that can't even defeat the rattata of some youngster we keep gouing first. Having a torterra or facing cynthia's spiritbomb are notable exceptions

I was thinking that maybe cpu pokemon have low IV or EV in general. the weird thing is the lack of information about the topic, maybe we just take it for granted during the playthrough.

We are usually higher levels leading to a higher speed stat and as for your point about Gengar, personally I usually attack second when fighting one in the games.

Yeah I'm not sure why thats happening to you. I mean I can say I usually attack first because a lot of the Pokemon I prefer are fast so it makes sense I would. A good example of why you are off on your theory though is when I played Black 2 recently. Going up against Skyla, I didn't move first a single time against her Swoobat and Swanna and my team included Leavanny, Lucario and Krookodile with a +speed nature. I was finally able to move first against here when I was 5 levels stronger than she was. Even with Pokemon known to be fast and one with a good nature, I was still second to move. Which was very deadly for me in that match.

So as you can see, we do not always move first as a player. Its just that we are typically higher in level and, most likely, gravitate towards faster Pokemon rather than slow, in general. Now I do not know about older games, if there were any glitches of some sort that made the player attack first sometimes even when our pokemon wasn't faster, but i suppose thats a possibility. As for the IV and EV stuff that's also a possibility and could account to a degree why it happens every once in a while but I strongly doubt that every single Pokemon has low IVS and EVS, especially in the newer games.

Is there any way you can give us a specific example of when this occurred, like for example a Torterra being faster than a Gengar? Not that that is even possible but it works as a hypothetical example.