I'm storing a lot of contacts in Outlook and I synchronize my mobile phone with Outlook. Because of all those contacts, it is hard to find the correct one on my mobile phone and is making my mobile quite slow. Most of them I'll never use on my phone anyway.
Is there a way that I can sync only the contacts that I use the most?
That depends on your phone. Some support filtering based on categories that you have defined in Outlook.
So for instance, you can assign certain contacts in Outlook the custom "Phone" category and you configure the sync application for your mobile device to only sync contact items which have the “Phone” category. Check the manual of your mobile device and sync application or contact your supplier to find out if your phone supports it as not all do.
Separate your Contacts in 2 folders
An alternative is to keep only your main/favorite contacts in the Contacts folder and store other contacts in another Contacts folder. As most sync software only supports syncing your main Contacts folder, you automatically filter out the rest.