Another train of thought:
1 - how much CPU/RAM does the system have?
2 - when did the slowdown first become noticable? Has it always been slow or is it gradually getting worse?
3 - how frequently do you update statistics and rebuild/reorganize indexes?
4 - how much memory is the max memory for SQL vs how much is the total memory for the system?
5 - how frequently does it autogrow and (heaven forbid) autoshrink?
6 - do you manually shrink the database?
7 - are the slow applications in house or 3rd party?
7b - if they are 3rd party, have you contacted support?
I would follow Gail's advice, but just a few other things to throw into the "why is it slow" troubleshooting pot.
I'd recommend hiring a consultant. They are experts in their field. They will be a lot faster and more focused support than a free forum.
That being said, your available MB in your first screenshot looks a tad low. I imagine putting more RAM into the system and/or configuring the max memory for SQL will have a noticable benefit.
The above is all just my opinion on what you should do.
As with all advice you find on a random internet forum - you shouldn't blindly follow it. Always test on a test server to see if there is negative side effects before making changes to live!
I recommend you NEVER run "random code" you found online on any system you care about UNLESS you understand and can verify the code OR you don't care if the code trashes your system.