After using your Android for few months, you might have noticed that there are too many duplicate entries in your contact list. This happens because of synchronizing multiple online accounts with your contact list. Each one of them will add a new entry for a contact. Some entries with the same email are merged automatically but still there would be some duplicate entries.
Same thing happened to me, it would take around 10-15 secs just to load the contacts list on my phone. I had around 700-800 entries in my contacts. So I think that was obvious. Manually checking and deleting each entry was not an option here. It would have taken at least 2 hours if I had to do it manually. So I went to play store and searched for some light and easy to use duplicate contacts remover in a hope that there would be at least one app for doing this odd job. But to my surprise, there were too many apps for this.
After trying 2-3 apps, I downloaded Duplicate Contacts to try and see if it serves the purpose. After installing and opening the app, it asked me to allow it to add a search shortcut to my home screen, add a bookmark and change my browser search settings, which I declined. But still it worked :D. After that it was all easy. It generated a list of duplicate contacts in few secs. I only had to click on two buttons. Show and delete.
It saved all my duplicate contacts in a vcf file on SD card before deleting. So that I can recover them anytime in future. That is one useful feature. Give this app a try if you feel your contacts list is too slow.