I'm working on that myself. I tracked a few things last night to see if I could find any patterns, but I think I need more data.
*Notice how these two games had similar times with the same XP payout. Could be something there.
**This game took a little over 5 minutes, but the XP payout was 75% of the Invasion games I played, which took almost 3x as long. If every Team Slayer game worked out like this one, I'd say it was the best place to earn XP efficiently.
I recall in classic Halo: Reach, a lot of XP was paid out for the time spent in game. Meaning you could (for example) play one Big Team Battle game or 3 SWAT games. Assuming either of these options took the same amount of time, then you would earn approximately the same amount of XP.
One thing I read is getting at least 10 kills will usually give you the maximum Performance XP of 8,000 XP. This seems to be mostly true in my tracked XP above.
I'll keep tracking/testing to see what works best, but so far it seems the best playlist is whatever you have the most fun in.