The speed difference is negligible.
HTTPS encrypts all traffic between the client and the server so that nobody in between can view the data being transferred. HTTP is in plain text, so all the data being transferred can potentially be viewed by a third party.
HTTPS is essentially a requirement for websites that perform financial transactions. It's also a good idea for sites requesting personal information.
An SSL certificate costs money. Typically, the easiest approach is to contact your hosting provider about it.