We have been troubleshooting this issue and believe it occurs for 2 reasons:
1) An extremely large number of plugins
2) Slow site performance
In the case of slow site performance,please report this to your hosting provider. It is often the case that when the performance issues are resolved, the slow backup issues are resolved as well.
We also recommend deactivating and deleting any plugins and themes that are not being used. Large numbers of plugins will not only make the backup slow, but your site as well.
In most cases this issue is resolved by removing unnecessary plugins and and in the case of slow site performance, when the host corrects the issue causing slow performance.
If the issue continues, please send us a support request via the wpbackitup site.
We will assist you with troubleshooting the issue.