Ext4 on /data will make loading big apps faster. But even with no lagfix (rfs), it doesn't lag like the stock kernels.
With rfs, *sometimes* when RAM is almost full and u switch between big apps, it is a bit slower vs ext4 when loading the app - so maybe u get 1-2 seconds of black screen while loading. But that's pretty acceptable in my opinion, because the battery life with rfs is better than with ext4.