Sources:
http://nexus.leagueoflegends.com/2017/09/upcoming-changes-to-leveling-ip-rewards/
Instead of getting IP after every game, you’ll receive an unlocked chest—called a capsule—every time you level up. These capsules contain champ shards and blue essence, and if you hit special milestone levels, you’ll earn special rewards like gemstones or hella rare emotes.
https://boards.na.leagueoflegends.com/en/c/general-discussion/nXlJYxE6-shout-out-to-cactopus-and-other-rioters?comment=00000001
The whole system is balanced so it will be as fast or faster to unlock champions than before with the IP-based system. That said, we are making First win of the Day more important (it'll give a ton of XP). So if you're the type of player who plays tons of games every day you might find it slightly slower. Otherwise you'll be unlocking champs in at roughly the same or faster rate.
https://boards.na.leagueoflegends.com/en/c/gameplay-balance/sJ8N14Hy-riot-cactopus-just-confirmed-we-are-no-longer-getting-ipbe-after-games-how-do-you-feel?comment=001b
We are NOT making it harder for you to unlock champs. Unlocking champs is getting way faster for everyone with preseason with runes becoming free, and then we're also tuning leveling speeds specifically so that leveling gets you champs as fast or a little faster than the old IP-after-every-game system.
https://boards.na.leagueoflegends.com/en/c/general-discussion/8h0YpEWl-how-to-reward-vets-without-just-dumping-30-boxes-on-them?comment=00150006000100010000000000000000000000000000
This is why we keep stressing that the new system isn't going to slow down the rate at which you earn champs. For players like you, you're almost certainly going to be earning them faster than before. The only folks who might earn champs a little more slowly are the people who play tons of games every day and reach a very high level. And even then it's only going to feel a little slower for them because we're putting more power into the First Win of the Day (it'll give a lot of bonus XP). So I don't think you have anything to worry about.
https://boards.na.leagueoflegends.com/en/c/general-discussion/8h0YpEWl-how-to-reward-vets-without-just-dumping-30-boxes-on-them?comment=001500060001
The chests you get in the new leveling system are almost ALWAYS champion shards, and that's not different for noobs.
The only exceptions are when you hit milestone levels (50, 75, 100, etc.), which give you exclusive cosmetic content. We definitely are not intending to give players exclusive cosmetic content for leveling from 1-30. There has been an idea floating around about maybe giving you like a random ward skin or whatever when you hit 20, but I don't actually think we're even going to do that. So again, if you're already level 30, you're not actually missing out on anything. You got IP when you were leveling. Instead of IP, noobs in the new system are just getting cheapo 450 IP champions and champion shards (which can be turned into BE, which is now the same as the IP you got back in the day).