It is an assumption but a quite logical one. The blocklist is saved server side.
If it is implemented well, that means, it has to be loaded once per session and then, on each request everything related to that list is checked against that list in your browser. Maybe more than once per request.
If it is implemented not so well, or if it is not possible to do it in the browser, then this calculation has to be done server side.
Im 99% sure that it is a size restriction for technical reasons.
And if you are for changes and not for regression, change the way you communicate with people and you dont need such a huge block list.
If it is implemented well, that means, it has to be loaded once per session and then, on each request everything related to that list is checked against that list in your browser. Maybe more than once per request.
If it is implemented not so well, or if it is not possible to do it in the browser, then this calculation has to be done server side.
Im 99% sure that it is a size restriction for technical reasons.
And if you are for changes and not for regression, change the way you communicate with people and you dont need such a huge block list.