In checkout when click on Place order button take time around 1 minutes .
Store > Configration > Sales > Sales Emails > General Settings > Asynchronous sending > Enable.
Then it take 45 seconds.
There are a lot of things that can slow down your website, email sending being one of them.
Magento introduced the Async email sending option just to avoid this kind of issues. Basically when you use this option, Magento, instead of sending the email when the order is placed, adds an entry in a queue table (
core_email_queue), and then a cron job comes, reads all the entries from the queue and sends the emails. I definately recommend using it.
Again, why it takes 15s to send an email is a hard guess. There can be multiple reasons, among which I can assume the email server takes to much to respond (in case you're using a STMP server)