Well shoot. Thanks for trying 😃. There isn't a hit-stop event or anything tied to attacking the skeletons so that's a head scratcher. No delay nodes. The health of the portal is depleted as the skeletons are killed and I had a health bar for it but it didn't really look like it belonged so I added the smoke effect everytime a skeleton died to show portal taking damage instead. The end screen should have said portal destroyed. I've tested it on 3 different systems and had no bugs. I'll need to do more testing. Appreciate your feedback! I'll add the health bar back as it's just a single wire in bp to set back up.
ETA: I believe the slow down was a bug with the health pickup compiling shaders when spawning. I've tried a quick fix for that by placing a health pickup out of the map to compile the shaders at runtime