Sending speed depends on various factors. Before continuing, it is a good idea to update your Sendy installation to the latest version.
Sendy only performs as well as the server it is running on. Sendy is a 'bulk email application', almost everything is performed in bulk, from importing CSVs to sending of emails. The larger your volume, the more server resources it uses.
The better the spec of your server, the faster Sendy will perform and the lesser timeouts will occur.
Although Sendy works on 'shared hosting', consider running Sendy on a 'VPS' or 'cloud server' for better performance or if you have a huge volume of subscribers.
If you can, increase your server's
memory_limit to a number close to what your server physically supports by editing your server's php.ini.
You have the option to select the Amazon SES region from which to send your emails, such as US East, US West, EU, and more. It is recommended to choose the region that is closest to the physical location of your hosting server where Sendy is installed. When you send a newsletter, Sendy will transfer your emails to Amazon SES, which will then send them out. Opting for a region that is closer to your server's physical location will result in lower latency and faster sending speed.
Sendy's MySQL database should be hosted on the same server where Sendy's files are hosted. Some users like to use external databases like 'Amazon RDS', however this is not recommended for Sendy.
Users who use external databases like RDS experiences slower sending speeds and unforeseen issues. RDS is good for 'storing' data but not optimized for quick real time data processing like what Sendy needs.
As Sendy transfers your email in bulk to Amazon SES for sending according to your SES send rate, depending on the capability of your server, your server may have trouble processing huge bulks of emails efficiently if your SES send rate is very high. Lowering your send rate may yield better sending speed and stability. You can lower your sending rate in Sendy's main settings.
Lastly, having your 'Sending rate' in settings unintentionally set as '1 email per second' would cause your emails to send very slowly. Do check your 'Sending rate' in settings to make sure it's set at the speed you want.
Size of your email's HTML
The size of your email plays a part in sending speed as well.
Your email’s HTML code is encoded with 'base64' before being transferred in bulk to Amazon SES for delivery. Base64 encoded code is 33% larger than the original code. Multiply this by your SES send rate in which Sendy base the number of emails to transfer per iteration, larger emails will naturally take a longer time to be transferred over to Amazon SES.
Attachments slow down email sending speed because of file size. Even though Sendy supports sending of emails with attachments, it is still recommended to upload your attachment to a file hosting service (eg. Dropbox), then include the download link in your email instead of attaching it directly in the email. You'll get better sending speed and your emails are less likely to end up in your recipient's spam folder. In other words, even if you are able to send through your email with the huge attachment, it is pointless if your email end up in your recipient's spam folder.
The 'queue' table where Sendy temporarily store records of sent emails when sending a campaign may be filled with lots of outdated records. Try emptying the 'queue' table when no campaigns are currently sending by visiting http://your_sendy_installation_url/clear-queue (replace 'your_sendy_installation_url' with your Sendy URL).
Also, rebooting your server will help free up server memory which may result in faster sending speeds.