Dear Rajneesh,
As you told that you have Samsung Galaxy Ace Duos with 400MB RAM and 850mhz processor but still the phone does not work as fast as you want.400MB RAM is not enough for bulgy applications and games to work fast.You should either avoid installing bulgy applications and games on your phone or use that slowed down phone.In my opinion 400MB RAM is not bad for small sized games and applications.High graphic games need much more MB RAM than your;s one.Avoid Installing phone boosters on your phone because many of them don;t fasten;s your phone but slows down It;s functioning.
Use SD card of large size and keep some free space In It.Example:If you use 8GB;s SD card then keep at least 2GB-3GB free space In It.Scan your phone on a regularly basis,because viruses can even slow down your functioning.Don;t download bulgy files from your phone Instead use your PC for that.Avoid plugging your phone to recharge for more than 3hours.Don;t let your phone;s blue tooth always on.Avoid use of China headphones and china batteries,because they could damage your phone.Instead use the brand which is recommended by your cell phones company.If you will take care of all the above measures then your phone will definitely work fast.
Your;s friendly,
UMESH