This site enables customers to upload their photos

Can One put kb/sec (or similar) limit on uploads to avoid them affecting performance from the frontend of my website?

Should you experience such strong performance impact, you need to split your site onto multiple servers, one which provides read-only content and also the other one that's used once the user interacts by uploading files or doing database updates.

Slowing down lower the upload (or download) bandwidth will really result in the impact elsewhere worse - not better before you really become so terrible where all of your incoming bandwidth can be used up.

Before you decide to seize around the caveat I have just given - this can be a very unusual scenario for any webserver. Go measure your bandwidth usage out and in first.

When the very remote possibility that the incoming bandwidth is totally hogged by uploading then you will need to stop people from uploading som much - not slow them lower to ensure that the procedure hogs memory, CPU and procedures for extended.

It is almost always smart to limit the connection rate (not bandwidth) per client address to safeguard against DOS attacks - but this really is better done in the perimeter of the network instead of around the server itself.