Hello Danny,
Runtime is probably a factor that matters, for the reason you mention (server load). However, browser should not matter when it comes to sending the results to the server (it does matter if, for example, you use the MediaRecorder or EyeTracker elements, which are poorly supported by Safari, if supported at all)
The most important factor regarding data transfer is the amount of data being transferred: if a submission contains thousands of lines, storing them in the server’s database will take time, and as incoming results accumulate, the risk of crash due to overload increases
Jeremy