Upload kills download on Shaw GPON

I switched from Telus GPON to Shaw GPON and I've noticed that whenever I'm uploading something that saturates the connection my download speed also grinds to a halt. Thought this was something Fibre should prevent from happening? I remember this being a huge issue with DSL and Cable but thought that Fibre would've resolved it as I never had this problem with Telus or Novus.