Aww come on, guys! "Undocumented", not "Illegal".
This is U.S. Immigration 101. Undocumented or not, we are all immigrants, right?
On a different note, I don't recall the exact percentage of participation after the last executive order, but it was nowhere near 100. In other words, even if 5 million people were eligible, there won't be 5 million extra petitions in the USCIS system. Moreover, it requires resources. Anyone applying for DACA will have to find and pay for a lawyer, pay for fees, get documentation sent from their birth country, etc. As many as of us know, this process takes time. Again, if Congress approves a budget that funds USCIS, there will NOT immediately be 5 million applications flooding the system.
So honestly, I wouldn't even even expect major delays in the processes of the "legals". But I do think it's important to keep an eye on the legislative process trying to block the president's initiative.