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Actually, the speed is not the problem. Everyone can cope with it. Besides, Lightning Network is present which could fix the problem 
. I think the main problem why bitcoin is still far away from the mainstream is because of technological illiteracy. Huge portion of our population are still mot aware about this so educating others would be very important factor to consider. Another reason is that people are into fiat, they feel contented with it already thus refuse to explore other means. Nonetheless, our community keeps on growing little by little and that's good.
I will add the scamming made where bitcoin has been used as their currency since for this it really create bad impression to the people as well with the legal entities, so before this adoption on mainstream maybe we need a good usage for this interms of merchant acceptances since if there's no something like this will happen and spread across the globe then maybe bitcoin will get a hard time for big adoption.