Making Stock Rom faster without root?

Hello, I'm using a Samsung Galaxy S GT-i9000 with Android 2.3.6 Gingerbread. 2 years ago, I made a factory settings on both internal storage and internal SD card, and only installed one app which is WhatsApp just for communication. I didn't load my Google account into it as I know Google Service will make the phone even more laggy. However the new WhatsApp updates make the phone lag to death. Now when I click into WhatsApp it takes 10seconds for the phone to respond. When I click into someone's chat it will show a black screen for almost a minute. I guess WhatsApp developers doesn't expect people using a single-core phone with such low Android version nowadays. Is there anyway to make the phone faster? It would be better if no root or custom ROM is required. Thank you :)

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