I think this applies to all forms of racing but I would guess lower HP class maintaining corner speed is key to fast laps, but how does one accomplish this? I feel that I am going as fast as I can but only mid pack overall laps. Is it chassis setup, tires, skill, something else or all of the above?
Drive faster and see if it holds.
You mentioned in the tire thread that you weren’t seeing the same gains on the new SH2 as others. This can be down to not fully exploiting the new grip that’s on offer or possibly chassis setup issues being exacerbated by the higher grip level. Very common for some drivers to drive to a certain level or comfort regardless of grip levels. If you are only driving at 90% of your kart’s potential, the grip level won’t matter because you aren’t at the limit, so you won’t see gains from the tires providing a higher limit.
The new spec tire feels very noticeably grippier on-track and all braking zones are deeper and apex speeds are higher, so if you aren’t feeling a big difference to the old tires I imagine you aren’t at the tire’s limit.
Initially it may help to further process where your losing time at. It’s possible it’s both on the brakes and in the corner itself, but it might actually be more one than the other. And those two things, braking and cornering, obviously have to come together for the big show, but for learning purposes how the kart behaves at and past the limit it is easier/better to work on them in isolation at first.
After choosing what to work on first, and before going out for each session choose 1 or two places on the track that you feel a) comfortable with and b) like you’re missing some pace and endeavor to purposely and gradually work up to exceeding all braking-points and/or adhesion limits you believe apply. In this way you will know that you have found the true edge because you’ll be planning to stick a toe across it. You’ll see how the kart reacts (just) past the edge of control, become comfortable with it, and likely surprise yourself at where the edge actually is. 
If your still struggling after all of that (e.g., you’re already at the limit of braking/adhesion) maybe you just need a different line, to jump some curbs more/less, or maybe a change with your karts setup.
Take speed off earlier and less, shorter. Earlier back to throttle and see if it holds. 