Most living beings travel the slow path, each day of their lives following the one before, an endless progression until their deaths. Subjectively, the passage of time varies; for some the pace of life is slow, unhurried, while for others it passes so fast that if they so much as blink they might miss it.
For Time Lords it’s different. Where others see time as a one-way street, to the Doctor and his fellows it’s more like pool. They can dip into it wherever they choose, but must take care not to cause too many ripples.
Time can be fragile.