Back in 2008, Gmail launched the “Always use https” feature just to increase the security of your email conversations. At that time it was not enabled by default because this feature if enabled has the tendency to slow down the data transfer across the web. But now Gmail has enabled this feature by default for everyone.
In https mode data you sent over the web is encrypted to make sure its safe from prying eyes. Since encrypted data travels over the web slowly as compared to unencrypted data, you might notice your mail to get a bit slower, but from security and reliability point of view you should keep this feature enabled.
Though it is recommended to keep “Always use https” feature enabled, but if you want, you may change the settings to disable it. Click on the Settings tab in the top right corner of your Gmail account homepage. Now check “Don’t always use https” in front of Browser connections.