Migration Assistant estimated time remaining keeps climbing

macOS progress and completion estimates are often based on the number of bytes transferred. But one large file will usually transfer much faster than many small files even if they add up to the same size. What may be happening here is that the transfer started with some large files, went quickly, and resulted in an optimistic estimate. But now it is into small files which take longer and so the estimate is growing.

The estimate and speed can also be affected by the type of storage, whether the migration assistant is trying to do other tasks at the same time, whether FileVault is enabled, the type of connection for the backup volume, etc. All of those factors can magnify the differences in file sizes as well as cause variability on their own.

Make sure both the backup volume and the new MacBook are plugged into their respective power supplies and give it some time. The worst case scenario is a hardware problem with either the backup or the MacBook, but give it a few more hours before jumping to that conclusion.