I have a Blood Elf Frost Mage that's level 19 and every time I level, my Frostbolt takes just a bit longer to cast. Right now, the normal cast time is about 2.1 seconds, and with a special ability it's about 1.5. Why is this?
Spell casting speeds are artificially lowered at low levels so you don't have to worry too much about advanced concepts like spell pushback and to make the low-level game a bit more fast-paced.
The cast speed is scaled until around level 20, where you'll eventually hit the normal casting time for the spell.
For example, Frostbolt's actual cast time is 2 seconds: you reporting 2.1 seconds at level 19 indicates you're likely at or near the end of the scaling.
Additionally, if you have any items that have the haste rating attribute, one thing to keep in mind is that while haste lowers casting speeds, the amount of haste rating required to maintain the same speed reduction increases with level. So if you don't swap out your gear for things that have more haste rating, you'll find spell casting times increase as you gain levels.