I want to synchronize data from excel file with SharePoint list on office 365. I have already built a utility which uses client object model to achieve this but it is very slow as item count are in thousands. Please explain if there is any other better way to achieve this.
SharePoint List Synchronizer is working for me to restore automatically the two-way synchronization feature between Excel and SharePoint online lists. I found it very reliable and easy to use!
Go through this link which shows you steps by step method
New link (but for 2010):