Firstly, I tried the "Clear Data" way. Did a backup in WhatsApp (Settings > Chat settings > Backup conversations) and also in Titanium Backup. Then went into Settings > Apps > WhatsApp > Clear data. Restarted WhatsApp but didn't felt any smoother.
Next, enabled the Force GPU rendering option under Developer options. Scrolling in WhatsApp did felt smoother than before. I toggled a couple of times to compare. Yes, it is smoother.
But that wasn't enough for me, I checked on http://www.whatsapp.com/android/ that the version is 2.9.2871, while mine was still running 2.9.1547. Attempting to download it from the website on my phone would just load Play Store, so I downloaded it on my PC and copied it to the phone and installed it. Then only, I knew what Holo theme meant :P It's smooth without enabling the Force GPU rendering option.
Before
After
