why MySql 8.0.17 Select query performance is slower than Mysql 5.7.26?

[Sudarshan m]

We have very basic query like

select * from Some_Table
which used to work fine in Mysql 5.7 . And same data in Mysql 8. 0 is taking some time double the time( around 2 million data). We have upgraded from 5.7 to 8.x and seeing the performance degrade. Also we tried changing the values for below system variables in my.cnf file but the issue still persists.

sort_buffer_size
read_buffer_size
read_rnd_buffer_size
myisam_sort_buffer_size
max_allowed_packet
wait_timeout
interactive_timeout
connect_timeout
Any help will be great , how to make 8.x faster than or equal to 5.7x