There are a number of parameters that can be adjusted within MySQL's configuration file that will allow you to improve the performance of the server over time. For initial set-up you may find Percona's my.cnf generating tool useful. This tool will help generate a my.cnf file that will be much more optimised for your specific server capabilities and your requirements.

Do not replace your existing my.cnf file with Percona's one if you have already loaded data into the database. Some of the changes that will be in the file will be incompatible as they alter how data is stored on the hard disk and you'll be unable to start MySQL. If you do wish to use it and you have existing data, you will need to carry out a mysqldump and reload:

mysqldump --all-databases --routines -u root -p > ~/fulldump.sql

Once the dump has been completed, shut down MySQL:

sudo systemctl stop mysql.service

sudo cp /etc/mysql/my.cnf /etc/mysql/my.cnf.backup
sudo cp /path/to/new/my.cnf /etc/mysql/my.cnf

sudo rm -rf /var/lib/mysql/*
sudo mysql_install_db
sudo chown -R mysql: /var/lib/mysql
sudo systemctl start mysql.service

sudo apt install pv
pv ~/fulldump.sql | mysql

MySQL Tuner is a useful tool that will connect to a running MySQL instance and offer suggestions for how it can be best configured for your workload. The longer the server has been running for, the better the advice mysqltuner can provide. In a production environment, consider waiting for at least 24 hours before running the tool. You can get install mysqltuner from the Ubuntu repositories:

sudo apt install mysqltuner

mysqltuner

-------- Recommendations -----------------------------------------------------
General recommendations:
    Run OPTIMIZE TABLE to defragment tables for better performance
    Increase table_cache gradually to avoid file descriptor limits
Variables to adjust:
    key_buffer_size (> 1.4G)
    query_cache_size (> 32M)
    table_cache (> 64)
    innodb_buffer_pool_size (>= 22G)

One final comment on tuning databases: Whilst we can broadly say that certain settings are the best, performance can vary from application to application. For example, what works best for Wordpress might not be the best for Drupal, Joomla or proprietary applications. Performance is dependent on the types of queries, use of indexes, how efficient the database design is and so on. You may find it useful to spend some time searching for database tuning tips based on what applications you're using it for. Once you get past a certain point any adjustments you make will only result in minor improvements, and you'll be better off either improving the application, or looking at scaling up your database environment through either using more powerful hardware or by adding slave servers.