The 75% build reduction actually translates to significantly less in-game. The majority of the construction time is spent on the engineers running from one corner of the s-mine field to the next, making the construction time less significant.
You do realize they halved the distance between the patches as well? The actual reduction in an entire field build time is pretty close to 75% even after accounting for movement.
You are not penalized in any way for only building one patch at a time except through being forced to manually give a command to interrupt building, so you can avoid even that little moving if you want.
The sweeping time on a s-mine is directly proportional to it's build time. It takes about 1 second to sweep the mine.. I don't know much much faster you expect it to be? The enemy spent 60 munitions building the mines - surely you can spend 30 munitions to clear them.
It's not, please take a look at how badly pioneers/engineers behave when you're trying to get them to sweep sometimes. They will often derp for several seconds before clearing a single mine, and with smines there are around 20 of those patches for every 60 munis spent so you're often looking at a >1 minute clear time for a field that takes <15 seconds to build. I've complained several times about the sweeping speed issues with soviet mines when playing as germans (in that a single squad lays them faster than the pioneers will sweep them at), and smines are several times worse than that.
And honestly it's not even about how long it ties up your engineers compared to how fast it builds. The real issue is that if a german pioneer squad gets to a cutoff point, they can make it completely impossible to recap without a vehicle or minesweeper in a couple of seconds.
s-mines are now on-par with Soviet mines and this change was one of the rare moments Relic actually proactively improved a useless ability.
I'd never make a soviet mine for anti-infantry purposes again if they had the current smines. The current smines also hugely penalize any strat that isn't conspam and allow for some pretty silly abuse with the extremely limited retreat paths on most maps.
Edit: I'm not sure why you said mortars aren't efficient at cleaning the s-mine field. They trigger from any damage source, regardless of the damage. This means a single splash of a mortar is enough to clear half the field.
They're honestly not. Not only is the accuracy of ground target all over the place, Most hits will only explode one or two mines at best. I'd always rather conspam than get mortars against anyone I know is going to abuse the new mines.