Zip library "Allowed memory size"

In my test, it worked well as long as you are not trying to create a huge zip. In that case, yes, the browser is going to jam while its waiting for it to complete. You could create a table where you add the request as a job and then check with a chron job every 5 mins or so and process the requests or as mentioned do it using "exec". not all servers have that enabled though so you would need to check that first.