Record clamp/weight and turntable speed

Simao, there are many things that can push the speed a little one way or the other such as temperature and AC line drift. Very few turntables are right on unless you can adjust the speed. Some modern turntables actually have a computer monitoring the speed and making the adjustments for you in unnoticeable increments.
@chakster , you are right. It is the tonearm and cartridge that produce the sound and DD turntables make them sound lousy which I can easily demonstrate to you in person. The last thing you want in close proximity to a cartridge is a pulsating magnetic device. Not to mention that I know of no DD turntable that is adequately isolated making matters even worse. But everyone knows by now how the two of us feel and neither is going to change the others mind without a demonstration that can not be performed. So it is a moot point.
Clearthinker Theoretically I agree. The more weight you place on a bearing, the shorter will be it's life span. (except for magnetic and air bearings). Threaded reflex clamps are the way to go for sure unless you have vacuum. However the bearings of most good turntables are very over-engineered and unless subjected to trauma you could measure their lifespan in centuries. My Sota Sapphire was as quiet when I sold it as the day I bought it 40 years previously. That is listening to the sub chassis with a stethoscope. I also agree on your bling thing. Turntables are meant to be heard, not looked at. They only need to be tidy and well built/finished, just one of the reasons I like Sota's and Dohmann's. Tricking one out to make it look like Star Wars is a waste of money, money that could be better spent elsewhere. 

Mijo, I am getting a little excited about the idea of a "pulsating" anything, anywhere in my vicinity, given the dearth of human to human contact that characterizes my day to day existence during the pandemic.  Anyway, I have to disagree with my DD co-conspirator, Chakster, on at least one issue.  I do think that each type of drive system has its own sonic signature.  Guess which drive system I prefer.  I also think you ought to see someone about your fear of pulsating magnetic fields.  You are surrounded by magnetic fields, unless you live in a cave.  But seriously folks, I bought an EMI meter just to satisfy my curiosity about EMI in the vicinity of the cartridge that might be generated by the motor of a DD turntable.  Detected nothing at the platter surface.  This was with a Technics SP10 Mk3 which sports the largest iron-core motor of any vintage DD and probably at least as large as that of any modern DD.  I also checked my Kenwood L07D, which uses a coreless motor.  Nada.  Of course this observation is subject to the sensitivity of my meter. The fact that you personally may fear the negative effect of a DD motor on cartridge performance is not per se proof that there is any appreciable effect in reality.  The engineers that designed DD turntables in the first place were not unaware of the potential problem and were aware of how to use shielding and motor orientation in order to eliminate the problem. They did a good job.  Now, can there be a cheap, poorly designed DD turntable that adds its own noise to the audio signal?  Maybe.  Can there also be a cheap poorly designed BD turntable that is wildly inaccurate speed-wise?  Absolutely yes.

@chakster    3kg???  Many TTs have platters weighing 25kg, some much more. Over-engineered glitzy TTs with extra bling are one of my pet hates.

I’m talking about additional platter mat and additional record weight, not about stock platter weight. 
Some modern belt drive weight a ton, looks like a bunch of heavymetal, cost like a house and still ugly as hell. This is what you’re talking about? 
A mass market belt drive turntables are very light and if you will put 3kg in the platter you will burn the motor probably. 
The weight of additional platter or clamp is irrelevant for Technics direct drive, no harm for the bearings and everyone knows it. Been using SP-10mk2 with Micro Seiki CU-500 mat and ST-10 weight (about 4 kg additional weight all together). Still immediate start with Technics DD motor. New SP-10R is even better! 

I have a slight twist on the OP's question, in that I have a 70's era Thorens 145 belt-drive turntable that I have introduced a center weight to.  As was pointed out, this weight does not affect the platter speed at all, it just takes a slight moment longer to come up to speed, so, no issues there.

However, on these Thorens tables the center spindle is not overly long, so, with anything at all other than the original rubber mat (which is quite thin) elevates the record to the point where there is hardly enough spindle to engage the hole in the middle of the weight (the weight I have also has a slight taper on the underside I suppose to help someone guide it onto a spindle).

I try to get it centered which helps most of the time but there are times when the weight is rotating a bit off-center.  It does not affect the sound at all, but, just curious if there is any other thing it might be doing?  

Also, some may be worried about VTA, but, we are talking a slight slight elevation here, not enough to change the VTA requirement.  Even with the factory mat on the table the spindle barely clears the center piece on that table (which is also the 45 adapter if turned over).  

Thanks for any thoughts or comments.

I wouldn't worry one second about the slightly off-center weight, but if the aftermarket mat is thicker than the original, and if you have set VTA while using the OEM mat, then the VTA will have been altered by a tiny amount.  It's up to you to decide if that has affected SQ or tonal balance.  Probably not, is my guess.  If you are using a cartridge with a line contour stylus, which is very sensitive to VTA, listen carefully.