In answer to your questions:
1) This can depend on your hosting setup, but the theme itself isn't know to be slow. If speed is a concern to you, there is plenty of WordPress documentation available relating to optimization which you may find useful, such as
this.
2) Whilst we don't officially support third party plugins, there are a couple of plugins recommended in
this blog post that have been successfully used with our themes.
3) AppThemes doesn't officially support customizations, so support is limited to installation/setup/configuration rather than modifications or code changes. Our support team endeavours to respond to forum threads within 24 hours.
4) As above, customizations are not supported, and we do not offer these services, however we do have a
certified partners page where you can find developers for hire who are experienced at working with our themes.
5) This would involve customization at this time. Thanks.