Lead lists start to get slow and buggy as they become larger in size. We're continually working to improve how the platform handles large quantities, but if you experience this issue you might want to try keeping your lists smaller and more specific. You could build them by city and job title (Marketing Managers in San Francisco for example).
Also, make sure you're not working out of the Master list. That's the one we put in there for you when you first create your account.
If you've been capturing and never created a new Campaign, you're probably capturing to the Master list. That list is intended to be a reference of all the leads you've ever captured, and as such will quickly become very large. Instead, create your own new lists for each new campaign or project.
If you have any trouble, please contact our Support team by clicking "Submit a request" at the top right of this page for further assistance.