Insert into table is slow in the night but very fast during day time
The data isn't in memory when it runs at night, based upon the PAGEIOLATCH wait that you mentioned, but is in memory during the day. Your Page Life Expectancy is probably taking a nosedive at night due to all of the other maintenance that is going on. Having to get the data from disks and especially slow disks is much slower than if it were in memory already. Seems like you're going to need to add more memory since getting the data pages from disk is so slow due to the I/O issues you are experiencing.