Powerup

Yes, power ups above level 30 are 1/2 what they were up through level 30. Look up a Pokémon's info on this site and at the bottom is a table of min/max CP's per level....and this effect is shown in the numbers.

After pokemons reach lvl 30, their cp gains are halved.

Under advanced Pokemon stats

https://pokemongo.gamepress.gg/pokemon-stats-advanced

you can find the table for CP/powerup

Level CP per Power Up
1-10 CP * 0.009426125469 / CP_Multiplier^2
10-20 CP * 0.008919025675 / CP_Multiplier^2
20-30 CP * 0.008924905903 / CP_Multiplier^2
30-40 CP * 0.00445946079 / CP_Multiplier^2

Level 34 and I hatched a Lapras last week. Got 17 candies and evolved several times. CP increased 41+ each time, Lapras now CP 1939 and I am waiting for more candies.

One thing that wasn't mentioned in this thread is that you can power up your pokemons up to your trainer level + 1.5.

So if you are level 30, you can power up your pokemons to level 31.5. But like everyone said, the power ups give half the CP after your pokemon reaches level 30.

When Pokemons are at level 30, they're basically close to max so powerup gain is half so you could take your time maxing it out once you get to 40.