Gasly isn't that slow. He is significantly slower than Max at this stage in his career but the pressure and everything else associated with moving to a new team under these circumstances is making him perform way below his talent level.
Red Bull need to be careful here as Kvyat was the first one to get promoted to the A team when he wasn't ready and it messed with his career. Gasly is struggling even more than he did and now faces a long road to recovery even if they give him another chance in the Toro Rosso. I think the longer he stays in the Red Bull, the worse it will be for him. Albon and Kvyat aren't ready for the big seat either.
Not everyone can be Vettel or Verstappen. The well is currently running a bit dry in terms of the Red Bull academy. Gasly or Albon could be a solid first team driver for them one day, but until that time comes, they need to consider swallowing their pride and putting a Hulkenberg or a Perez in the car for a year or two. Sainz would actually be the perfect person for them right now as he is performing well for McLaren and is one of their own but the timings didn't work out for them last year for whatever reason.