What portion of the difference in the angular speed before and after you increased the mass can be accounted for by frictional losses

As the mass increases, the moment of inertia(I) increases, therefore, the angular momentum(L) increases too.

Explanation:

friction can be defined as resistance in motion of bodies in relative to one another

momentum is the product of mass and velocity

torque is the time rate of change in momentum

τ =

where L = Iω = mvr

I = moment of inertia

ω=  angular frequency

if there is no external force(torque) acting on the system, then

= 0

dL = 0 = constant

moment of inertia I depends on the distribution of mass on the axis of rotation.

as the mass increases, the angular momentum(L) increases

angular frequency, ω, remains constant

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explanation:

see earlier answer