Poor fractioning of my Phones "Tecno Spark 4"

Been using the tecno spark 2 for few years and never experience any problems at all, then I wanted an upgrade of Tecno spark 4 which we bought a month ago and we both started to experienced problem with the new phones, thought is mine alone but my partner is experiencing the same problems. I really don't know what to do it freezes while typing, calling, googling the net, sometimes the key pad takes long to appear when you need to texting and very slow. I am so disappointed as I thought it will give me the same quality as with the previous spark 2 but I regret buying them, worse we bought two at once at Mr. Price Boksburg East Rand Mall