Thanks for reaching out to the Community with your question! We are aware that some of our customers are experiencing slowness at night, and I have tracked that this is happening to you. We are working on figuring out why this is happening, but it seems to happen only when using our Legacy Editor.
Right now the workaround is to use a template created for our 3rd Generation Editor. You may find this to be a good time to make the switch as we are actively retiring our Legacy Editor, and soon the copy option will not be available on Legacy Emails.
To get started with a 3GE template please navigate to your Campaigns page> click Create> Email> select a new template. You can learn more about making the switch by clicking the link below.