Note that the Spaces plans will only let you host resources (either individually or zipped) but you won’t be able to send data from your experiment, ie. DigitalOcean Spaces are not compatible with the MediaRecorder element and the EyeTracker element’s data-collection method.
There are many factors that impact resource preloading. One of them is the bandwidth of the host server, so if you store your images on a fast server, they might load faster than from the PCIbex Farm (especially if the farm’s servers are busy). Using a ZIP file is actually likely to slow the initial preloading a little bit, because the experiment needs to download all the resources at once (the zip file) before it can preload any of them. But it is a more optimal option overall, because (i) one zip file is lighter than the total of all the resource files fetched individually, and (ii) it requires only one request to the server, when multiple concurrent requests can in some cases incur significant slowdowns.
If preloading time is not that much of an issue, keep in mind that you can always store resource files (but not zip files) on free services like Google Drive or Dropbox (you just need to use a direct link, not a “share” link)