Not sure exactly why it's so slow, but looking at the requests it makes, it looks like they're sending an invalid value for the "until" filter. Also, the main ui doesn't load until all off the relays have connected or failed. There probably something that could be done there to improve their responsiveness.
Discussion
Yes, it must be a relay problem with #zapddit. Because it loaded when I only had one relay in the NIP-07 plugin cache database. I then added 8 more and then it stopped functioning. So it does look like a little bug they need to fix.