High mega pixel camera problem

I acquired a Canon EOS 5DsR to replace an EOS 5D classic. I was puzzled with many un sharp images produced. Camera combination was 5DsR + 600f4+ x1.4 TC mounted on bean bag on a window sill of a stationary car. Camera was at Av and A modes. EXIF data indicated that the bulk of the un sharp images were at1/1000 sec and below.
I recently stumbled across a snippet in an article that mentioned that this is not uncommon with images produced with high megapixel cameras because of the high density pixels on the sensor, 50+mp on the DsR. Solution, use higher shutter speeds. No guidance on minimum shutter speeds to be entertained to avoid camera shake and ASA noise.
Grateful for guidance and thanks.
I recently stumbled across a snippet in an article that mentioned that this is not uncommon with images produced with high megapixel cameras because of the high density pixels on the sensor, 50+mp on the DsR. Solution, use higher shutter speeds. No guidance on minimum shutter speeds to be entertained to avoid camera shake and ASA noise.
Grateful for guidance and thanks.

The high pixel count is very unforgiving and ANY camera movement is likely to be recorded. Upping your shutter speed is definitely one way to go but also ensure that your camera technique is faultless. E.g. rest your arm on top of the lens to help prevent any movement - use a kimball and tripod where possible, back button focus, shoot in short bursts (the first image may be soft as you depress the shutter release, but the second image will likely be steadier, the third may be soft as you release the shutter) or use a remote shutter release. If you subject will allow consider mirror lock up or live view shooting (the mirror is up when using live view). The maximum focal length I use currently with my 5DS is 600mm @ f5.6 (a 120-300mm f2.8 zoom with x2 converter). The more I have practised the higher my hit rate and I have used the combination successfully handheld.

The whole high MP causing soft images thing is a red herring IMHO, the pixel density on a 7D MKii is greater than a 5DS R, and is a very popular wildlife camera, technically the 7D MKii is a higher resolution camera than the 5DS R, though the manufacturers marketing departments won't agree.
The autofocus system is worlds apart from that on the 5D MK1, I understand that the cross-hair focus points are only effective on lenses or extended lenses with a maximum aperture of F5.6, which I suspect will hit you with a limitation on your lens extender combo.
The autofocus system is worlds apart from that on the 5D MK1, I understand that the cross-hair focus points are only effective on lenses or extended lenses with a maximum aperture of F5.6, which I suspect will hit you with a limitation on your lens extender combo.

Just come a similar comment on soft images produced with high mega pixel cameras in U Tube talk. See link below:
https://www.youtube.com/watch?v=KHzfanLhUNA