Flamestrike
Legend
This may interest you: if you do the math to on Leveling Up (PHB 15) with Adventuring Day XP (DMG 84), it's supposed to take about 34 "adventuring days" to go from 0 to 20.
Level Experience Point Total XP to Reach Level (delta) Adjusted XP per Day per Character Adventuring Days Total Adventuring Days to Reach Level (delta) 1 0 0 300 0 0 2 300 300 600 1 1 3 900 600 1,200 2 1 4 2,700 1800 1,700 3.5 1.5 5 6,500 3800 3,500 5.74 2.24 6 14,000 7500 4,000 7.88 2.14 7 23,000 9000 5,000 10.13 2.25 8 34,000 11000 6,000 12.33 2.2 9 48,000 14000 7,500 14.66 2.33 10 64,000 16000 9,000 16.79 2.13 11 85,000 21000 10,500 19.13 2.33 12 100,000 15000 11,500 20.56 1.43 13 120,000 20000 13,500 22.3 1.74 14 140,000 20000 15,000 23.78 1.48 15 165,000 25000 18,000 25.44 1.67 16 195,000 30000 20,000 27.11 1.67 17 225,000 30000 25,000 28.61 1.5 18 265,000 40000 27,000 30.21 1.6 19 305,000 40000 30,000 31.69 1.48 20 355,000 50000 40,000 33.36 1.67
Awesome work!
That largely mirrors my experiences; I feel the 'extra' speed at levelling from 12th+ comes from the fact that higher level PCs are more often dealing with singe 'High CR' Legendary monsters that give a greater XP 'yield' than does a small group of lower CR monsters (which are more common at low to mid levels).
6-8 low CR monsters have the same adjusted XP as does a solo High CR monster, but the solo has the higher actual XP yield.
'Teen' level PCs would deal with a high CR Dragon or Pit Fiend (with some legendaries tacked on) or a Lich for example, and the XP from just 2 or 3 such encounters is often enough to 'Level' them during T3 and T4 play.