If your only currently known symptom is "some SQL statements are slow", then you need to start with the usual analysis steps even before you get to troubleshooting.
1) What is "slow"? Why? Did it used to be different? How long ago? What is a list of everything that changed over that time?
2) What is "fast"? Why? Is that even reasonable (i.e. a full table scan of 25TB of data in 0.05 seconds is not reasonable... no matter how much someone wants it)
3) What hardware are you on right now? Keep coming back to this to get more detail, down to the particular spindles and what shares each element, as you work through the other aspects, until it's "fast enough".
4) What wait states are growing fastest when it's "slow"? "fast"?
5) What SQL statements are involved? All? Some? What do they have in common? Time of day based?
6) What else happens when it's "slow"? When it's "fast"? On the database? On the instance? On the hardware? On the network? On the SAN? On the user's machine?
There's a lot to look at and a lot to learn - any one "quick fix", whatever it is, will only work if that addresses most to all of your current issue. You may have a different current issue, or more than one.
Alternately, free up some budget and hire one of the many experts available, some of whom post here.