The kind of security warning you’re getting can’t be changed in settings as far as I know. Memory is the most likely culprit. Every frame code can put a lot of load on memory, see this thread: Update every frame doesnt work
It may simply be that older computers don’t have the memory for it. You could advise people to close as many background programs as possible, but unless you can find a way to optimize memory usage further, this may not be readily fixable. @jon or @dvbridges might have further optimization advice, but as far as I can see you’ve done most of the obvious things (reduce file sizes etc.).