Performance is obviously relevant in the margin. A performance improvement in your site might decrease dropoff or churn by something like 1-5%, which is a good number, and it will compound.

But if you are still at the stage in product development where adding features will increase your addressable market by tens or hundreds of percents (which doing things like ‘supporting multiple European languages’ or ‘adding google login’ can do) then those performance gains are not where engineering investment needs to go.