Just my personal anecdotical evidence: I am using /e/OS on my own FP3+ for four years now (by now on Android 15) and compared with another FP3 that ran on Android OS (formerly called Fairphone OS) Android 13, the FP3+ runs smoother and snappier. This might be the case with other Custom ROMs as well, however Android OS will certainly provide the highest possible compatility with apps.
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