The time it takes to perform the initial update is mostly drive I/O dependent. 25TB is a huge amount of data. If you assume, generously, that a USB 2.0 drive can read data at, say, 50 MB/sec, then if you were to read each drive in sequence, 25TB would take about 139 hours, or 5.8 days to read it all. Now in the initial update it reads from all drives simultaneously, which should be faster than reading them in sequence, but you are also writing to the parity drive, which will compete with reading to slow things down. Overall, 7 days doesn't sound too bad for USB drives. You are almost certainly running up against the limits of your system's USB bus.
On my system with 14TB spread across 10 SATA drives, the initial update takes about 20 hours. Keep in mind that the more drives you have, the slower parity builds because the more drives it has to read from simultaneously (i.e. each 1 GB of parity contains 1 GB of data from each drive, so if you have 12 drives, it has to read 12 GB of data to generate 1 GB of parity.) If some of your drives are smaller than others, you may find the parity build rate will increase over time, since it has to read from fewer and fewer drives the farther along it gets.
I can't predict whether switching to USB 3.0 ports will help. Maybe a little, but probably not that much.
Anyway, the good news is, once your initial update is done, subsequent updates will be fast. When you add new data, it only has to read the new files you added to update the parity. It doesn't have to start over from scratch each time.