I have a revelation of sorts for 5DSR users - I'm going to jump to the punchline and just get it over with - buy a grip for the camera and grip it (assuming you are handholding the camera) - it makes a huge difference for stability and lowering required SS.
The 5DSR has been a love hate camera for me - I love the pixel density - it drives me mad how high SS are required to get those tack sharp images (repeatedly, not 1 offs - but repeatedly 90% of the time - big difference)
I basically settled almost exclusively using the Sigma 24-105 F4 OS because for some reason, the weight of that lens perfectly matches the camera - and you can shoot it at 1/40th at any FL and get repeated tack sharp pictures at F4 - the only other lens that you can do that with is the 100-400L II - in my opinion, maybe the best IS lens on Earth - and the weight and the mass of it perfectly dampens the camera so that the combo just works.
The update that led me to the revelation about a grip is getting, but first testing repeatedly, in store, and many copies at that, the new Tamron 85m 1.8 VC. First it was finding the absolute correct MFA to dial in the lens - compared to back to back to back to back against the 1DX II that nailed it every time, the 5DSR would still hit and miss - at a 50 megapixel test. I then stumbled upon using AI Servo and highspeed continuous as being slightly better than One-Shot and single silent - for whatever reason this is true - again, things getting better more consistent - it is so much lighter than carrying a 70-200 F2.8 II - it feels wonderful - plus you can drop to F1.8 (I prefer F2) and get those wonderful bokeh portraits.
But then i decided today to really test out the lens versus the magnificent Canon 70-200 F2.8 at various SS, all handheld, to judge the ability to get tack sharp images - the Canon f'ing murdered the much lighter Tamron in consistency - that simple - it wasn't until 1/160, but really 1/200th where the Tamron final became its equal - at least at this point you are 1 1/3 stop faster and all is good -but it still pissed me off to know end.
Since I am now a 1Dx II owner, I was bringing this camera into the mix - and it nails the Tamron every single time even at crazy low SS. And then it hit me, it is a mass thing, a balance thing - the camera needs more mass - just like that 1Dx II - the camera's size in your hands and that little bit of extra added mass has to be helping.
I own a 5D3 grip but never use it - ever - why would you - makes the camera heavier for no reason - except, maybe it might work for the 5DSR and - holy fvck - it is worth at least a stop - probably 2 - and all of a sudden the SS and the successful hit rate on the 5DSR with the 'lighter' Tamron is exactly like the 1DX-II - i.e. it works.
If you don't own a 5DSR, or maybe you aren't as crazy as me in terms of owning something and wanting it to be able to deliver what it has promised but has somehow not been able to do - trust me, grip the camera - it helps.
For me, prior to today, I have had success with the Sigma 24-105, the Canon 70-200 and the Canon 100-400L II - no other lens am I able to consistently use and get tack sharp images handheld - it just doesn't 'consistently' get tack sharp at anything that doesn't approach 1/5X FL for shutter speeds.
After I gripped it, I can now take 1/40, even 1/20th tack sharp images with the Tamron 85 1.8 VC - and do it repeatedly (that is the key).
If when I mount the Canon 70-200 or 100-400 or Sigma 24-105 I will not grip it - it is not needed, but with the Tamron, which is a much lighter lens - increasing the mass and size of the camera body completely changes the consistency, and in a dramatic way, of being able to use 1/40 - 1/125 shutter speeds and for those pixels to be in tack sharp focus.
Hockey and wedding photographer. Favourite camera / lens combos: a 1DX II with a Tamron 45 1.8 VC, an A7Rii with a Canon 24-70F2.8L II, and a 5DSR with a Tamron 85 1.8 VC. Every lens I own I strongly recommend [Canon (35Lii, 100L Macro, 24-70F2.8ii, 70-200F2.8ii, 100-400Lii), Tamron (45 1.8, 85 1.8), Sigma 24-105]. If there are better lenses out there let me know because I haven't found them.