When the number of users increases, the site is slowed down I have a database with 25 M record in 42 tables(all tables is Myisam). When the number of users increases, the site is slowed down . pleas help me .

Server Config: Linux , Centos
Ram : 30 GB
CPU : 20 Core

my.cnf parameters:

[client] port = 3306 socket = /var/lib/mysql/mysql.sock

[mysqld] port = 3306 
socket = /var/lib/mysql/mysql.sock local-infile=0
 skip-locking skip-innodb
 wait_timeout=50
 max_connections=550 
query_cache_type=1
 query_cache_limit=1M 
interactive_timeout=50 
connect_timeout=10 
max_allowed_packet = 128M 
join_buffer=1M 
table_open_cache = 1024 
sort_buffer_size = 30M 
read_buffer_size = 30M 
read_rnd_buffer_size = 8M 
key_buffer_size = 6G 
bulk_insert_buffer_size = 256M 
myisam_sort_buffer_size = 246M 
myisam_max_sort_file_size = 4G 
myisam_repair_threads = 1 
myisam_recover thread_cache_size = 16 
query_cache_size = 960M
 max_connect_errors=10 
record_buffer=1M 
table_cache=2048 
thread_concurrency = 40 
log-bin=mysql-bin
 server-id = 1 


[safe_mysqld] err-log=/var/log/mysqld.log open_files_limit=65535 [mysqldump] quick max_allowed_packet = 128M

[mysql] no-auto-rehash

[myisamchk] key_buffer_size = 512M sort_buffer_size = 256M read_buffer = 16M write_buffer = 16M

[mysqlhotcopy] interactive-timeout
MyISAM.  Say no more.  The "table locks" are probably what causes the performance to decline as you get more 'users'.

Some tips on the conversion to InnoDB:  http://mysql.rjweb.org/doc.php/myisam2innodb