The characters moving into a time that runs faster than the source is always to their benefit, it’s why it doesn’t matter if living memory is still running faster than the source or has aligned because it only benefits the protagonists relative to source time

If the WOL had travelled on the train and did or didn’t summon the scions they would have fixed the problem of Alexandria in seconds by source timekeeping

The only problem they could run into with time dilation is if they move to a shard that runs slower than the source because then time spent in the shard would be amplified on the source