Server slowed down when having high session count

Story. we change domain of forum. then bots starts reindex. But sow whats too much CPU usage on mysql process..

And found this - > much sessions slows down mysql .

SELECT * FROM phpbb_sessions  where session_browser = 'Mozilla/5.0 (compatible; YandexImages/3.0; +http://yandex.com/bots)' or session_browser = 'Mozilla/5.0 (compatible; YandexBot/3.0; +http://yandex.com/bots)';

where was 257000 rows

then add new bot with partial "Yandex" and delete all bot sessions.

CPU goes down by half.

GRAPH CPU usage blue.

where was about 120 request/s via workbench viewer.

Have suggestion? maybe need keep up to date some popular search engine bots?