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Developer mode isn't really something you should turn on just to speed up your phone. I've been on the road for 25 years, my phone is a tool to stay connected, and I've seen people try to optimize Android in all sorts of ways. Developer mode lets you limit background processes and disable animations, yeah, that'll speed up response a bit, but it's not a magic bullet. First you should figure out what's actually slowing your phone down.
If you still want to try it - go to settings, find "about phone" and tap the build number a few times until developer mode turns on. Then a separate menu will open in settings. You can disable window animations and app transitions there, that'll give you a noticeable effect. You can also limit background processes, but don't overcomplicate it - leave it on normal limits, otherwise apps will keep closing.
Honestly, it's better to just clean up your phone - delete unnecessary apps, clear the cache, maybe reinstall the system if things are really bad. I had my Wi-Fi at the dacha lag once from tons of apps, reinstalled it - and everything worked. That helps way more than any developer mode nonsense.
I've got Developer Mode turned on my phone, but not for speeding things up - I disabled the animation for screen transitions there, and the interface actually does feel more responsive. But that's not magic, it's just removing unnecessary glitches. If your phone is lagging, the problem is usually a cluttered storage or some app running in the background eating up resources, not something you need to mess with in Developer Mode. I've seen people randomly tweak stuff in there - like limiting background processes or turning off sync - and then complain that something broke or stopped working.
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