I have around 800,000 observations.

Here is a sample of the data. I simply want to compute the first lag; for
example the lag value for 006-2000q4 would be 156 and 128 for variable 1
and 2 respectively. Any suggestions will be appreciated.

ID    yearqtr  variable 1  variable2
006   2000Q1   100         130
006   2000Q2   150         180
006   2000Q3   156         128
006   2000Q4   158         789
006   1999Q1   456         759
006   1999Q2   369         789
006   1999Q3   599         789
006   1999Q4   589         456
005   2000Q1   225         789
005   2000Q2   596         489
005   2000Q3   369         589
005   2000Q4   598         877