Such slowness is usually fixed by MySQL config tuning and by identifying and optimising slow queries.
For the latter, turn on the slow query log, put the system through its paces, run Explain on the queries it's identified, optimise them. For help with such a query post the query, the Explain result, and the Show Create Table result for each of the query's referenced tables.
For system tuning and identifying the overall degree to which query optimisation might help, please post the amount of RAM, the MySQL version, and ...
(i) post the result of ...
select engine,data,indexes,total
from (
select
ifnull(engine,'TOTALS') as engine,
concat(data,' GB') as data,
concat(indexes,' GB') as indexes,
concat(tot,' GB') as total,
if(engine is null,-1,tot) as ord
from (
select
engine,
round( sum(data_length)/1024/1024/1024, 2 ) as data,
round( sum(index_length)/1024/1024/1024, 2 ) as indexes,
round( sum(data_length+index_length)/1024/1024/1024, 2 ) as tot
from information_schema.tables
where engine is not null and engine not in('information_schema','performance_schema')
group by engine with rollup
) sums
) list
order by list.ord desc;
(ii) post the results of running these queries in the mysql client program ...
show variables;
show global status;
if mysql 5.7 or later ...
select * from sys.memory_global_total;
select
substring_index(event_name,'/',2) as code_area,
sys.format_bytes( sum(current_alloc) ) as current_alloc
from sys.x$memory_global_by_current_bytes
group by substring_index(event_name,'/',2)
order by sum(current_alloc) desc;
select
concat(format(a.num * 100.0 / b.num,2),"%") bufferpoolfullpct
from
(select variable_value num from performance_schema.global_status
where variable_name = 'innodb_buffer_pool_pages_data') a,
(select variable_value num from performance_schema.global_status
where variable_name = 'innodb_buffer_pool_pages_total') b;
... and look for runaway mem use in the results of ...
select *
from performance_schema.memory_summary_global_by_event_name
where event_name like 'memory/performance_schema/%';
(iii) from the OS post the result of ...
free -m