MySQL causando over load no servidor

Pessoal muito boa tarde,

Trouxe mais um desafio .

Tem um banco de dados com centena de milhares de linhas e colunas.

Cada usuário logado no site faz uma média de 300 consultas de vez , isso porque tem bate-papo entre outros que demandam vários selects.

No entanto quando tenho um usuário on-line a CPU do servidor do BD fica em 0% , agora quando aumenta , tipo 30 pessoas on-line (conectadas simultaneamente) a CPU vai para over-load.

top - 14:19:04 up 14 days, 20:24, 1 user, load average: 4.17, 3.52, 2.82
Tasks: 287 total, 1 running, 286 sleeping, 0 stopped, 0 zombie
Cpu0 : 77.8%us, 4.0%sy, 0.0%ni, 12.9%id, 0.0%wa, 0.3%hi, 5.0%si, 0.0%st
Cpu1 : 54.5%us, 1.7%sy, 0.0%ni, 43.9%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu2 : 24.3%us, 0.3%sy, 0.0%ni, 75.4%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu3 : 27.0%us, 1.3%sy, 0.0%ni, 71.7%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu4 : 46.8%us, 4.0%sy, 0.0%ni, 49.2%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu5 : 9.3%us, 0.3%sy, 0.0%ni, 90.3%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu6 : 41.5%us, 1.3%sy, 0.0%ni, 57.1%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu7 : 0.0%us, 0.0%sy, 0.0%ni,100.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu8 : 45.3%us, 1.3%sy, 0.0%ni, 53.3%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu9 : 16.3%us, 1.3%sy, 0.0%ni, 82.3%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu10 : 12.7%us, 2.0%sy, 0.0%ni, 85.3%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu11 : 21.3%us, 1.7%sy, 0.0%ni, 77.1%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu12 : 44.2%us, 0.0%sy, 0.0%ni, 55.8%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu13 : 6.7%us, 0.3%sy, 0.0%ni, 93.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu14 : 7.7%us, 0.3%sy, 0.0%ni, 92.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu15 : 3.3%us, 0.0%sy, 0.0%ni, 96.7%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 8057796k total, 3081888k used, 4975908k free, 154520k buffers
Swap: 5242872k total, 0k used, 5242872k free, 1681408k cached

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
23360 mysql 20 0 8857m 946m 5992 S 464.6 12.0 145:19.35 mysqld
555 root 20 0 0 0 0 S 0.3 0.0 43:35.11 jbd2/dm-0-8
2368 zabbix 20 0 75612 1792 1392 S 0.3 0.0 6:02.90 zabbix_agentd

Aguardo ....