Rootbound? Repotting time?

That plant is still on the small side for that pot size.. you are fine. rootbound is when there's a bunch of roots wrapped around so much that it starts to restrict water. You want enough roots to hold the medium together, along with some moisture while you pot up so it doesn't disintegrate during the process. There's a shit ton of time between that point and becoming rootbound. It won't accidentally happen under any normal and common sense circumstance. you'd probably help yourself more by constituted the medium differently. I don't see much aeration/drainage amendments. A high-water capacity base will need as much as 50% of volume and something that holds 2/3rds the water, like coco, will need 33% for the best gas:water ratio per volume. I prefer vermiculite, but perlite and other options are available. Not too chunky and not too fine... also the constant changing of its environment indoors and outdoors is probably not helping. if it is slow growth, it's not the pot size that is at fault. potting up is not stressful, but don't do it whimsically, either. The best way to determine, in my opinon, is how fast it drinks the water down.. .but that also assumes you are employing proper watering practices, in general, too. How fast it drinks the water-weight down is directly related to root mass. Early on in small pots, i'm not transplanting until it's 1-2 days between irrigations that are a reaction to dryback (can perceive through weight loss) and not some pre-ordained watering schedule regardless of context.