I have been looking for free or paid json data feeds on currencies for long.
I have come across:
The problem is that I really need millisecond based updates.
Can someone advice me on the way forward?
If you want millisecond updates, you need to use technologies other than json.
There are so called mid-rate reference price providers who eventually offer newsfeeds, however, the answer will very likely be a commercial service.
The otehr path would be to contact liquidity providers and / or exchanges directly and ask for API / feed access , which again , will most likely be a commercial service.
https://Polygon.io has FX real-time and historic APIs.
WebSockets for real-time updates, and RESTful API for historic data.