So I bought some used linear rails. The bearing blocks look well made, internally they are machined very well with a smooth and shiny finish and do not move side to side when on their rail.
The problem I faced was that the whole lot was not lubricated when I got it, so I lubricated all of them with white lithium grease and now they move very well and feel good (it's the only measurement I have) except only when moved at a moderate to high speed. If I try to move the block slowly, the bearings inside buckle and almost feel like they are jamming.
The blocks, ball bearings and their rails were cleaned thoroughly and the blocks were packed with grease well and extra grease was squeezed out during assembly. The part of the rail where the ball bearings slide do not show any signs on damage.
Can someone explain why this is happening and how do I fix it?