Realistically, the best way to know is to run the SQL in a test environment and measure the results.

I would assume using a default would cause the alter table to run slower as it has an impact on inserts. When you perform an alter table in MySQL on MyISAM, it creates a new table underneath with the new column and inserts the data into it and swaps them out.

So if it slows inserts initially and speeds selects, you'll see a similar performance. It could impact bulk load performance however, but you may prefer to break that load into chunks instead. I don't believe the alter table is using a bulk command for this and as such, would not reap those benefits.

What would be best is to create a new table with the additional column, insert the records from the original table, then rename them. I have also read using Percona schema toolkit is another good method to use here.

StackOverflow and MySQL references:

SO users on altering columns.

Percona suggestion.

Another Percona and other tool reference.

MySQL reference.