There is a big difference. You're only considering the "slowness" from the application perspective of querying a table without the index.
You need to also realize that database server itself takes a load hit when it kicks off these operations on large tables. I'm not sure on PostgreSQL, but I know sometimes you can not immediately cancel these operations in the middle on MySQL (it also takes time to revert).
With gh-ost you have full control over how fast this process goes and can even pause/resume it if you're experiencing issues.
But let's say it IS instantly cancelled. You're also ignoring the fact that you will take the same hit on any replicas if this statement is successful.
Some production environments don't take kindly to having all replicas lagging as the replication thread is blocking on the DDL change. My team has the luxury of being able to have our entire production environment served by a single master server (though we avoid it as much as possible), but it won't be long before we outgrow that and require at least one up to date replica. Many teams are already in that situation and for that, gh-ost is a godsend.
You need to also realize that database server itself takes a load hit when it kicks off these operations on large tables. I'm not sure on PostgreSQL, but I know sometimes you can not immediately cancel these operations in the middle on MySQL (it also takes time to revert).
With gh-ost you have full control over how fast this process goes and can even pause/resume it if you're experiencing issues.
But let's say it IS instantly cancelled. You're also ignoring the fact that you will take the same hit on any replicas if this statement is successful.
Some production environments don't take kindly to having all replicas lagging as the replication thread is blocking on the DDL change. My team has the luxury of being able to have our entire production environment served by a single master server (though we avoid it as much as possible), but it won't be long before we outgrow that and require at least one up to date replica. Many teams are already in that situation and for that, gh-ost is a godsend.