When you're going for exp, go for mp0 unless your damage is high enough that mp1 is just as fast and you're one-shotting that trash. If you really want to check the beginning of stonefort for the keywarden for that 5% chance at a key, that's okay. But it's not your focus when you're exp farming and you shouldn't go far looking for him. If he's not the first enemy you encounter, leave. Ideally you shouldn't even make this check, but it's not a huge slowdown if you do. As Xopht mentions, efficiency is key, so get into the habit of skipping things like regular chests that just slow you down.
When you go for keys, bump to a higher mp (3+) and make those runs specifically for keys, get 5 stacks, kill the keywarden, repeat. If you try to compromise on exp and keys by running something like mp1 when you're not really ready to farm mp1 for exp, you'll just make yourself less efficient at both.