Turntable weight affects speed

It just may not have the power to haul the extra weight. Is the belt clean / new? Have you tried giving the platter a hand in spinning up to speed?

Agree with @noromance.

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From the manual:

TD 850
Drive System: Thorens Belt drive
Motor: Fully electronically regulated AC-Synchronius motor

If the belt is clean/new and the bearing is lubed then the problem could be the added load on the AC motor and or regulated power supply caused by the weight.

FWI, Increase the load on an AC motor current goes up to maintain HP, speed. Current adds heat both to the motor winding as well as the "Fully electronically regulated" AC power supply. (The speed of a AC-Synchronius motor is controlled/maintained by the AC frequency.)

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How heavy is the weight?

My weight (TD125 MkII) is a 45 adaptor attached to the flat side of a medium-small brass Mapleshade cone (3 tiny spots of poster tack) and the total weight is approx. 8 ounces.

The DIY weight is as symmetrically balanced as I could get it (pretty easy as the flat side of the cone is the same shape/size as the 45 adaptor) and it’s non-magnetic.

I’ve tried heavier commercial weights with various mats and the 8 ounce unit is all that is needed as I heard no improvement with heavier weights (sometimes the opposite - mine is a "sprung" design).

I did not experience speed problems though within the weight range I experimented with.

I was initially concerned about increased main bearing wear and strain on the motor with heavier weights.

DeKay

If the weight is too much, try a clamp, like the SOTA clamp.  It really improves sound quality, particularly clarity.