I can understand how people really want a higher level cap and I can certainly see how, in early access, leveling up might feel too fast. However as has been said before, this is by design. Not by design of Larian, mind you, but by design of DnD 5e.
For your convenience,
here is the experience table of the game on roll20.net.
As you will see, the amount of experience required for each level is not linear - it increases as you level up. That is why getting to level 3 is pretty easy. That is very important too, since many classes get to choose some kind of path or specialisation at level 3, which will have a huge impact on how the class plays and feels from then on out. At that point there also is a significant increase in EXP required for level 4. The increase is even more steep from level 4 to 5 and so on. So while leveling up from level 1 to 3 does feel fast because it is, it will naturally slow down afterwards.
TLDR: Patience. Leveling up will most likely feel just right, come full release.
I don't think, that the level cap will be raised before then, especially since we don't even have all classes, yet. Implementing the remaining classes with the level cap in place seems logical as opposed to raising the level cap first and implementing the remaining classes afterwards.
