Does 'Group By' Affect The Query Speed?

i have a table such sa below:

Name1, Name2, Name3, Nam4, C1, C2,.., C100

and in this table, i have found index for Name1-Nam4,

i don't why sql below is very slow?

select
Name1, sum(C1), ...., Sum(C100)
from
(
select
Name1, Name2, sum(C1) as C1, ...., Sum(C100) as C100
from
(
select
Name1, Name2, Name3, sum(C1) as C1, ...., Sum(C100) as C100
from
(
select
Name1, Name2, Name3, Name4, C1, ...., C100
from
My_Table
group by Name1, Name2, Name3, Name4
) as T
group by Name1, Name2, Nam3
) as T
group by Name1, Name2
) as T
group by Name1



Does 'Group By' affect the speed of query?