It is at a shorter distance (say 25' or less). Many manufacturers incorrectly label their cables as high speed certified, even though the certification was for a shorter distance. When in doubt, ask the manufacturer for their certification certificate. Check it to see the length of the cable that was certified.
However, remember if you attach anything to your splitter that is stereo (such as the TVs), then the source devices will only send stereo. So, it may not be the cable.
As to how I would do this, that answer has changed over the years. Now I would run a 35' Redmere active HDMI cable. Before I would have said to get the lowest gauge 35' standard speed HDMI cable you could find. High speed just means you can run at the maximum bitrate that HDMI uses or foresees. Standard speed means it is certified for at least 1080i/720p but can do somewhere up to high speed. Where it stops, depends upon the cable. That is why it is very smart you are testing before installation. What I think you have now is a 35' standard speed cable.