Is it normal that X axis requires a bit more strenght than Y axes to move ?
From my experience with my Vertex a definitely no. I had to fine adjust the x and y igildur bearing mounts with a small hammer(hit from inside outwards at X axis and from the outside inwards at Y axes) so that they are most perfectly aligned to the rods. When I did that my print head noticeably moved smoother than before, and the x and y axis take pretty much the same force to move by hand.
Regarding that, how much force is typically required to move it by hand? My two axis’ are pretty similar but I think they require quite a bit of force to move by hand. The steppers have no problems moving them, but still…
It’s really difficult to tell since I don’t have a newton/force-meter. Doing this with only with a spiral spring and try to compare that to a weihght force on this spring…fussy.
When you are unsure I would recommend to remove the belts from the motors and untighten the belt holder/tensioners at X and Y slide bearing holder and then fine tune for yourself. Theoretically when the sleeve bearings entered slide mode there must be almost no friction. So if you tilt the whole printer by let’s say around 45° angel they will move by themselves pulled by g-force. Probably you will have to give them a little push so they can enter sliding friction mode.
When belts are disconnected the head should slide very easily, practically no resistance to hand should be noticeable. If it is stiffer than that, either the head slide block distance is incorrect or they are not parallel. If head gets stiffer near ends, adjust the distance. First determine whether distance is too long or too short and adjust accordingly. I determined this by pressing lightly the guide axle inwards/outwards while moving the head. If pressing inwards helps, distance is too short and so on.(Use glowes to prevent rust).
Adjusting is a bit tricky but doable. Be careful not to break anything, don’t blame me if you do! 
this is what I did:
To stretch I placed a steel rod against slide block inner surface and hit that with another steel rod sharp but not hard! Just enough to overcome the friction on the block.
To decrease distance, i put a piece of plastic in between slide block and frame plate, held other hand firmly against other frame plate and gave a controlled open palm smack on the other frame plate… Actually I think it was an closed arm punch but anyway. Perhaps something more rigid, wooden hammer for example might give a little more precision!