Speed sucks!!! please help

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http://forum.xda-developers.com – I have a single sim st21i, non-rooted. I don't have much apps installed but the speed has become really stinking. Only recently I had to reset my phone, to gain the stock speed ,but after just installing a few essential apps, it became sluggish. I tried apps like memory booster... but they are not effective. I am really speed concerned and it irritates me to have to wait 15 seconds flat to open apps like whatsapp... Can you suggest me what to. I will be grateful. Thanks: ) (HowTos)