that's how the parity building process works. i'm not sure there is any way to avoid it. for why it has to be like that i don't know i just know that's how it is
yes parity checks will take longer the bigger the parity drives are. it doesn't matter how much bigger they are than your data drives it's just based on the drive size of your parity drives. the size of the data drives do play a small part of how fast the parity building/checks go as the data drives are involved in the process but if say your parity is 10tb drive and your data drives are 4tb, 6tb, and 8tb, then all 3 drives will be involved until parity has checked up to 4tb then only 2 of the drives involved until the 6tb mark and then only the 1 drive until the 8tb mark and then only parity itself until it's done at 10tb