Slow Outlook performance may be caused by, but not limited to:
A very large Inbox, larger than 50,000 messages, or a large message store file. In earlier versions of Outlook PST files were limited to 2 Gb, however, more recent versions do not have this restriction but performance is still affected when the PST becomes large.
Poorly configured or loaded Add-Ins.
Poorly configured Outlook environment
Insufficient quantity of Random Access Memory (RAM) installed in the computer.
Slowed hard disk access.
On a regular basis archive messages from your default message store (the one actively receiving messages). The frequency of archiving depends upon the number of messages received in a given time period. For NEO 2.5 and NEO 3.0 (standard) users, this will make the archived mail messages inaccessible. NEO Pro 3.0 and the Unified Message Store functionality make multiple message stores available in a single catalog. Click here for more details on NEO 3 and NEO Pro 3 features.
Always have Outlook running when you start NEO, and keep Outlook running while you are using NEO. You can ensure this by going to NEO/tools/options and checking the option to start NEO when Outlook starts. In this way Outlook can stabilize and have the Add-Ins properly loaded before NEO is started.
Increase the amount of Random Access Memory (RAM) that is installed on the computer. RAM is relatively inexpensive to aquire and will greatly improve the speed at which applications can operate.
To speed disk access time, on a regular basis perform a disk defragmentation. This places files into physically contiguous clusters on the disk surface, allowing software applications doing a great deal of disk access to run faster.