Advise on reducing the load time of site.

I am looking for somebody proficient in php, mysql, and general server management to make the necessary modifications to the site attached to this project (included is database structure & php elements), and/or provide the appropriate advice that will result in faster load times. Essentially, the site this code is run on receives up to 300,000 uniques per day, each of which result in an entry to the DB, and when the DB has >60,000 entries, the site begins running slowly (4 core 3.5GHz taking >8 seconds to load, or timing out, because ~100% of the CPU is being used by mysql). I do not know if this is something to do with the coding of the database/site, or whether the sql connection limit is causing a backlog of requests, however once the CPU is fully loaded and the site is running slowly, if I truncate the tables, speed returns to normal (<1s average). I have a suspicion that there is some fix which will result in me not requiring such a powerful server to host a 500kb web page, despite the high volume of requests. Hopefully someone to provide insight on how to go about minimising load time, because all I keep reading is that the amount of entries in the database should have a negligible effect on serve/write speed. I’d imagine this isn't a particularly time consuming task, so bids should probably stay under $50, but if you know what needs doing, and it requires more time, then just say so and we can sort something out! I’ll not be back on Freelancer until about 8 hours after posting this project, so I’ll answer any questions anyone has then! Thanks, Harry PS, please include the word ‘monkey’ in your bid so I can separate the bottled bids from real people :)