Customize and Optimize an existing site/script

Hello, I currently have a website up and running that I need a few customization... First thing's first: I have a fiverrscript software software installed it with a few modification... now.. I am experiencing a few trouble with this script thus I need help to correct them... For now I only have 2 requirements to be done.. but it will be an on-going project so yes... if you did it right the first time... I can assure you that I can re-hire you for another requirement... ============================================================================== First requirement: ============================================================================== I want the site to load extremely fast... Under 3seconds (based on gtmetrix) ... now.. I already have done my research and there are a few things that can be done (I even have tried a few of them like page caching/CDN/cloudflare and etc...) but still there is a main problem with this script/software... I believe this is because the developer of the software/script embedded a proxy verifier or proxy blocker I have no idea what they call it... but there is something in the MySQL query that is slowing down the entire site and I want that fixed or removed... and lastly... I need the site to load UNDER 3SECONDS BASED ON GTMETRIX ============================================================================== Second requirement: ============================================================================== I need the default payment gateway to be replaced.. Currently the script/software uses paypal/payza and I want these to be replaced by a different merchant... I cannot publicly post the merchants name here but you will be notified which merchants are those... so yeah.. replace paypal with 1_another_merchant and replace payza with 2_another_merchant... and I also wanted this to work in the admin area... because I have done a few things like this project before and whenever I wanted a few changes... the things I wanted does not work/appear in admin area... so I have to manually do/check them via FTP or PHP file... That would be all... Thanks!