I’m afraid that’s what you get for that price! To improve performance, change to a better plan, or if you’re serious, upgrade to a VPN so you can tune the db/php/web server separately.
The front-end optimizations aren’t going to help - your first byte time is what is killing you (backend) at 20+ seconds.
Someone else was recently (within a week) diving into issues with godaddy first byte performance problems so it’s possible that they have an issue going on but support wasn’t very helpful.
You might be able to improve the performance by tuning the drupal app (enabling page caching, eliminating unused plugins) but if Godaddy’s servers are overloaded or having another problem your only choices are to have them look into it or switch hosting.
One thing that might help would be to look at your Google webmaster tools crawl stats (Health->Crawl Stats) and look at the “time spent downloading a page” to see if it has always been this bad or if things deteriorated on a specific day. If you can track it down to a point in time where you didn’t make any changes then you can use that information to hopefully have Support identify and fix the issue.
The above is definitely worth re-reading. In my experience, when ever in the past I saw a sudden big upward movement on the “time spent downloading a page” graph, it was always an indication of some technical issue (server-related issue).