What browser are you using?
For example if you use firefox and set up DuckDuckGo as your main search engine it will use HTTPS by default. Firefox DuckDuckGo Plugin
This are the instructions for firefox, chrome, opera & safari .
The great thing about DuckDuckGo, is that you customize it to your need (settings). You can specify there under Privacy Settings to use HTTPS by default. At the end it tells you how to keep the changes (Load/Reset Settings).
You could follow the same instructions on this answer and would increase DuckDuckGo security, but IMO it would be a bit overparanoid.
Post is generally faster, and since DuckDuckGo isn't as popular as Google they want to leave a good first impresion (assumption), thus fast and effecient results is the priority. User can add HTTPS later on as their default search option.
HTTPS needs a initial handshake which can take some time, if most of the data is sent through the initial request, performance will not be that significant.
SSL may decrease performance, in a way; Search results can be large spaning pages (take in account duckduckgo provides scrolling rather than clicking on next page) this might burden the system to encrypt all of this.
Also take in account, not everyone need HTTPS, even though it is recommended to use HTTPS.