save datagridview to a textfile

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VB

i have 2 datagridviews

while working both have a very large data about 70,000 + rows with 3 columns in each datagridview

now i have to save both datagridview to a single file with my own extension

so when user simple double clicks a file with my custom extension , my form1 is opened and data is filled to both datagridview again

now i have created such code where i am tacking single rows and writing it to the textfile
it is good for small number of rows but as lines added time is also increased .

for 70,000 lines it tacks about 10 mins , now user can not wait until 10 mins

is there any way to make it fast ?