Why is mySQL very slow after importing a lot of data?

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http://serverfault.com – I have been developing with PHP and mySQL. The report I use (TCPDF) is running well and doesn't have any problems. But after I try to import a lot of data, the report is not run and is blank. I Can't generate to PDF and mySQL is very slow I have been optimizing my tables and not changed anything. Can anyone help me? (HowTos)