Occasionally X1 Professional Client interacts with anti-virus programs, causing performance issues.
In some cases, you can improve performance by setting your virus scanner to not actively scan the folder used to store your X1 index. This will prevent your virus scanner from scanning the large files used to store your index (these files are constantly written to and read from X1).
You will still be protected as the actual virus scan on files/email/attachments has not changed (assuming that you have not turned off other features in your virus scanner).
The X1 program folder is located here by default: