did you speedtune your team? Have a look at Deadwoodjedis pages to have an overview about speedtuned teams.
You should have Deacon as leader and I think you should aim for about 315 end speed for Demytha...the rest is lower...
Haven't really looked into speed tuning atm, never seem to get much gear with speed stats. Currently got her at 230 speed with a little room for improvement.
You definitely want to be working towards something like this: https://deadwoodjedi.com/speed-tunes/myth-heir-salad/
But, the piece you're currently missing is Seeker, and while it is technically possible to do with Deacon instead, the requirements to make it work are absurd.
You could try this in the interim, if you have Doomscreech: https://deadwoodjedi.com/speed-tunes/myth-heir-with-no-seeker/
Alternatively, if you want another team while waiting on Seeker, you could try this one: https://deadwoodjedi.com/speed-tunes/mashalled-white-whale/