Use a loop to import, that way the button can click.

The real problem is the freeze. Use loops of 200 or under, make sure to
release all of your objects regularly, don't use dot operators that will
create internal variables that will not be released until the procedure
finishes.

If you are running against Exchange you can also run into the 256 RPC
channel limit so small loops are good for that.

Using SetColumns to limit the number of properties that are loaded is a good
optimization. Just remember to use ResetColumns at the end of your
procedure.

There are other optimization tricks but for that we'd need to know the
language you're using.