Despite being someone who can give up easily myself, I do feel you're giving up rather easily here. I'd try finding the reason for that lag, seeing how it can be improved, etc... at least to learn from the experience. Stackoverflow is a good place to ask about those things... despite a lot of their admins being a-holes who like to close your question for every stupid reason imaginable, if you ever go there be careful how you word everything when asking.

That being said, my general feeling is that trying to write a game engine fully from scratch may be a waste of time at this date, especially if you're planning to do it alone. Simply because there are already many good engines (including FOSS ones) ran by teams just large enough to maintain them properly! You can make a good engine of course and it's surely a good learning experience, however the time may be better spent using one that exists and matches your needs.

Just as much though I'd advice using and supporting one of the FOSS ones, not proprietary engines masquerading as open-source like Unreal or Unity. I'm still planning to better learn Godot which is great for both 2D and 3D... it's pretty much the Blender of the game dev world, I'd definitely give that a go sometime and see if you like it! Or if you want to make a HTML based game (my preference for simple stuff) I'd look at new technologies like AFrame which is very impressive. There's also PyGame (what Tuxemon uses) if you wanna easily code a game in Python

Godot is great, especially it's ability to export to HTML5.

I know it seems I'm being quick to give up, but I've actually been at this thing for about ten years now, have barely anything to show for it, and what I do have, I have honestly no idea how to do better. I've been having my faith in this chipped away for so long.

I also started this because I knew SDL would run on everything. I didn't want to be limited by engines that either chose to support only some platforms or put out games with high requirements. I wanted to be able to support little indie handhelds and such, like those OpenDingux machines.

I also tried Godot, and it's a huge, unintuitive mess to me. The hello world tutorial for the thing is a mile long, and it starting with a 3D view and no obvious way to just switch to 2D kind of intimidated me.

Oh, that is quite a bit of time. I thought it's an engine you started working on within the last year. It makes more sense then.

Yeah; Godot is complex to learn, similarly to Blender. It has a lot of features, and it's also rather new so the developers didn't have time to make the interface more user friendly. I imagine it must be an amazing tool once you learn to get past the complexity, similarly to how Blender is.

I'm curious what you can do with PyGame: That always seemed like a simple yet powerful toolkit to build something on. That would be one I'd suggest giving a try to.

Be more proud that you got THIS far than sad that you didn't make a functional engine on your first try. It's not an easy thing to do.

don't give up!! this seems like a small problem. i imagine the lag could be coming from tile collision testing; there can be many thousands in a level. but we can take advantage of how these tiles never move. a solution for that would be to store tile positions in a hashtable, then every time the player moves to check only the nearby spaces for tiles.
if the problem is something different, I wish you best of luck, should you choose to continue! I strongly hope you continue ❤️❤️