I got the impression that Heller dragged it out for so long because he didn't know what to do with Red John or who he would be.

http://www.tvguide.com/News/Mentalist-Simon-Baker-Red-John-1073837.aspx

TV Guide Magazine: What does this gruesome triumph do for Jane? Often when we get what we want, we find out it's not what we wanted at all.
Baker: "Protect me from what I want!" My wife has that saying on a little plaque in our kitchen. That's the catchphrase I took to [series creator] Bruno Heller and said, "This is what we have to do as the show moves forward. We have to deal with the comedown, the disappointment, the loss of focus." It's that meandering feeling you get after you've achieved the very thing that's driven you for years. Okay, you got what you wanted. What now? 

Baker: You want to know something I've never talked about?
TV Guide Magazine: Hell, yeah!
Baker: All those times we heard the voice of Red John? That was me.TV Guide Magazine: Seriously?
Baker: Yes! In the Season 2 finale, when Jane was captured and Red John did that "Tyger, Tyger" thing with the mask on? That was me. I played both characters in that scene.

TV Guide Magazine: Wow. That's especially trippy considering there was a major theory going around that Jane was Red John. So in a way that turned out to be true!
Baker: That was the reason I did that. There was a point where Bruno was saying, "We don't know who we're going to cast to play [the masked Red John]." I said, "Don't cast anyone! I'll do it." Bruno was, like, "I love it! I love it!" So I put on a mask and did that funny, wispy little voice. And for seasons after, whenever Red John spoke, I had to go record the voice.