Mysql Alter table add column with ALGORITHM=INPLACE, LOCK=NONE Am running below query

Query

Alter table dbname.tablename 
add column column1 varchar(50),
add column column2 text , 
ALGORITHM=INPLACE, LOCK=NONE;


It takes more time to execute.

Ideally running this without ALGORITHM=INPLACE, LOCK=NONE takes 30mins to execute.

Query am running for table size=24.4G and records-count=97lkahs.

Any suggestions on why does it take more time to execute?

Is there any way that i could achieve better on the same?

Version= 5.6.40
, Engine = Innodb
, Buffer pool size = 44.8 GB
Found out the issue. Ran out of storage space on RDS while running Alter table query. 

For some reason it did not throw error disk space issue.

But when i tried without ALGORITHM=INPLACE like the query below, it throws out error of disk space issue.

Alter table dbname.tablename 
add column column1 varchar(50),
add column column2 text , 
LOCK=NONE;

I increased 100G to see if that suffices the problem, it worked.

But i still did not understand the fact why does the alter query take 60mins to add two columns when i run the Alter statement above.

While Alter statement to add single column takes 30mins to complete.

How can we improvise the execution time for Alter statement, any suggestions would help?