Speed sensor issue?

So I went out to Santa pod today and I noticed an issue with my speedo, seems to accelerate up as normal and read the speed but if I am to slow down it has like a weird lag lol, theoretically if I was doing thirty and stopped quickly to a dead holt it would take a couple of seconds or so to catch up,

any idea? I'm thinking the sensor is sticking but before I pull it apart has anyone else go any ideas? it's a facelift so just has a electrical plug on top of the sensor
 
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