I've decided to make a dedicated thread for this as the discussion about the issue over in the years-old "Sky Sport Now - 2019 changes" thread has really gotten lost in the noise (and dredged up numerous times throughout the 37 page history).  It is my hope that someone technical from Sky can finally comment on this issue to shed some light.

Disclaimer: This is only a theory of mine which I have derived from my own experiences using both services over the course of about a year.  The devices I have used to come to the conclusions that I have include Chromecast, Chromecast Ultra, Samsung TV, Sony Android TV, Google Chrome, OnePlus 6 and Samsung Galaxy S10.  From other comments in the "Sky Sport Now - 2019 changes" thread, it would seem that the best possible experience can be had with Apple TV; a device I have NOT tried.  However, for reasons I will explain, I do not believe that these issues can possibly be device specific and I am more inclined to attribute the positive experience from Apple TV users to the superior app, and most likely the lack of an alternative service to directly compare against at any given point (such as Sky Go or Kayo Sports).  Put another way, I have no doubt that there are many happy Sky Sport Now users who are blissfully unaware that they could be getting an even better quality stream from Sky's own "Sky Go" app.   Furthermore, I must state that I am no longer a subscriber of either of these services so I cannot say whether things have improved or not.  Certainly, given the recent complaints from existing users it would appear that nothing has changed.

The issue
Sky Sport Now streams appear to have a lower frame-rate than their Sky Go equivalents.

The theory
Like any good streaming service, both Sky Sport Now and Sky Go offer multiple streams for the same broadcast of varying bitrate and quality to cater for all internet speeds, as well as maximum device compatibility.  This means that any specific device (for example Apple TV) might favour the highest quality stream whilst another device (for example an Android phone) might only max out at a lower bitrate stream.  As conditions change, the video player might switch to a lower or higher quality stream dynamically during playback, so it is not always obvious which quality you are currently watching.  However, it is possible to interrogate the video stream and retrieve ALL the possible quality options, and then manually force playback at that quality.  The easiest way to do this is to use Kodi, and a third party addon for SSN and Sky Go respectively.  Before playback begins, it first queries the stream and finds out which qualities are available (regardless of device) and then asks which one specifically you want to watch.   In both cases, we can see that SSN and Sky Go both offer maximum quality streams in the region of 8mbps, with a 1080p resolution at 50fps.  Again, whether you can actually watch at this maximum quality will depend on your device and your own internet speed, but the streams are there.

So what's the problem?  Both are 1080p50, right?  Well, not exactly.  1080p50 in this case describes (in part) how the video was encoded.  This doesn't guarantee that the original source video was 50fps to begin with.  It also doesn't guarantee that the source hasn't been incorrectly encoded.  I could record a video at 720p 10fps, and then stream it online at 4k 60fps if I so wished.  Anyone consuming the stream would see 4k60 in their player and be very disappointed with the result.  This, in essence is what appears to be happening with Sky Sport Now; when compared with the same channel / event on Sky Go at the same resolution, Sky Sport Now consistently has a lower frame rate.

Surely the source material from Sky should be identical?  Well, these two services use entirely different stream providers behind the scenes.  Sky Sport Now is an evolution of their old Fanpass service, whilst I believe Sky Go is a newer product (I stand to be corrected on this though).  My best guess is that the backend streaming provider Sky is using for SSN is butchering the encode.  Whether Sky is aware of this remains to be seen as they have declined to comment thus far.

The best I can do is try and prove my theory.  To that end, I created the video you see below.  Manually select the highest quality you can, and make sure it is one of the 60 fps options.  What you are seeing here is both SSN (left) and Sky Go (right), side-by-side.  This was a live broadcast with both streams running next to each other in Kodi on my desktop.  In both cases, you are seeing the maximum quality stream available (~8mbps, 1080p and a reported 50 fps).  You should quite easily be able to tell the difference in the framerate by watching the tennis ball and the players.  If you change playback speed to 0.25x, the difference is even clearer.

https://www.youtube.com/watch?v=UF2Ls4O20Po

One final disclaimer from me.  I think Sky Sport Now is a good service; I want to love it and I have subscribed on-and-off on three separate occasions since it was launched in an attempt to love it.  But having used Sky Go and Kayo Sports (Australia's equivalent offering), I just cannot commit.  On a large TV, it is simply not pleasant watching the sports I do with SSN and it is my genuine hope that this thread can raise some awareness so that Sky fixes it.