Website slow during several process

Hello I own a multi vendor website and there i have a tool where each seller can import products. Whenever a seller tries to import a product the whole website slows down and no pages open up. Only contact if you have worked on something similar before. So i have 2 issues here: 1) Right now the product import works only if import window is open. Once closed it stops working. I want it to work on sever side so even if window closed it still works and buyer can open the page again to see progress. 2) When it imports the whole website slows down. I want to increase the website speed optimised for all time and also especially when the import product script is working. I get many works done regularly on my website and if i like your work i will definitely re-hire you.