Turntable weight affects speed

@carlsbad2     "the load on the belt/motor should be the same with or without the weight" (INCORRECT physics). 

You are possibly thinking of Newton's Law of conservation of momentum.  But if the platter is spinning and you cut the power, it won't spin forever.  Ergo it takes power to keep it spinning.  How much power?  I clearly takes more power if the platter + record weight is heavier as the moment of inertia is greater.  Pace @testpilot , the point you make is correct but there will still be more inertia and this may be sufficient to prevent the motor on the Thorens keeping it moving at the same speed.

Yes, check out the motor, belt and clean and re-oil the bearing

Belt drive players are not designed for extra weight on them.  Oh, they will handle 200 gm records just fine, I mean extra weight.  Something to be said about record camps, too.  If you are seeking smoothness, fine.  For one who wants any "jump factor" forget it.

Try the "Origin Live Gravity One".....You will be amazed at the improvement in all aspects of the sound quality in particular soundstage and instrument separation as well base definition. Just my 2 cents and I do have heavy weights too so I can actually compare!

To add more context to the discussion, the OP’s platter weighs 4kg and the record weight is 291g - a 7.3% increase in mass which is positioned right over the axis of rotation.  I highly doubt that very slight increase in rotation inertia is causing the ac motor falling out of synchronous speed. I would look at lubrication and belt wear as the potential cause.  

Assume:

Without the added weight the TT platter RPM speed is correct when playing a record.

Add the weight the platter RMP speed slows down when playing, listening, to a record. ("It noticeably slowed it down.") Remove the weight platter RPM speed is correct when playing a record. Therein platter is up to normal RPM running speed.

My guess is the added weight puts more pressure on the spindle and the bearing it rides on. Therein added friction, drag.

I guess if the belt is worn the added friction, drag, caused by the weight there would some slippage around the motor pulley... Slower platter RPM speed.

One way to see if the motor RPM is slower with the added weight would be to check the motor pulley RPM speed playing a record without the weight and then again with the weight. If the motor pulley RPM is "noticeably" slower then it is not belt slippage. My guess then would be the "Fully electronically regulated AC" power supply can not supply the added power, (P = I x E), needed to maintain the power and frequency needed for the correct RPM speed of the AC-Synchronius motor.

At that point I would not use the weight!

.