Connection speed mystery

I get my internet as part of a tv/phone/ internet package from Virgin fibre. The speed I pay for is their 200 Mbit and I have been used to getting about 215 or thereabouts. For the last few weeks the pc than gets used the most, my beta one has been getting about 90 Mbits. The whole lot is hardwired with cat6, from the router, up into a bedroom and into a powered 6 way splitter and 2 go through into my computer room. WiFi is switched off as I am wary of its security in a built up area.
By chance, I happened to check the internet speeds on the new build pc on the other side of the room and to my surprise, the speeds were 220/21. The cables are separate, so I found a length of spare cable to reach across the room and the higher speed was running on the beta machine. It was back to the lower speed as soon as I reconnected the original cable. I did try swopping about the connections to the splitter and that didn't make any difference. So the individual wire was to blame for the slow speed.
I have just unwound 20 feet of new cable and was about to fit the first end on before feeding it through the wall, when I thought I would have one last try for speed. By magic, it is back obove 200 without doing anything new.
I have just rerun the test and got 224 Mbits which is a new record for me.

How many joiners/splitters in the original cable I wonder, each one will have a negative impact. Some companies do like you turning on and off the router and if you do it will start searching different speeds to find the best.

The cable is one piece and not being kinked or crushed, and I have unplugged both ends more than once to make sure it is not a poor connection. I had turned the router off and back on to reset it last week without any difference. Until this accidental result, I was also about to take my laptop downstairs and connect it to the router with 6 feet of cable to make sure of the speed before complaining to Virgin, that won't be necessary now.