Slow loading of some sites (e.g. apple.com)

Recently, some websites, notably apple.com, load VERY slowly through my Helix Internet connection. Other sites load fine. I am using the default Internet settings on my MacBook Pro with automatic DNS.

I think the issue is with Videotron's DNS servers, because if I switch from automatic DNS (i.e. Videotron's default servers) to Google, Cloudflare, or OpenDNS, everything loads much faster.