My query is taking too long to finish for finding the pair of rows where the difference of columns value is maximum group by another column

Question

Say, I\'ve a table like this:

I want to find the pair of Centers whose Performance difference is highest for each session, like this:

I have th

Answer 1

select distinct(session) * from (
select  t1.session, t1.center, t2.center, 
(case when t1.performance > t2.performance then (t1.performance-t2.performance) else (t2.performance-t1.performance))as performance_diff 
from mytable t1, mytable t2 
where t1.session=t2.session and t1.center!=t2.center) as T1 order by session,performance_diff desc limit 1;