The most plausible explanation I've seen is that Apple isn't being purposefully malicious and releasing updates to make older phones slower, but that they don't

The most plausible explanation I've seen is that Apple isn't being purposefully malicious and releasing updates to make older phones slower, but that they don't actively test the older phones with the updates and add features that take more computational power, causing the issues. I do think that iOS 11 has been significantly less tested (or built by less experienced engineers) due to the glitches and issues that have happened, but again I don't think this is malicious, just poor QA processes or major changes due to the release of iPhone X.