Migration to new MacBook Pro 16" from Time Machine disc stuck.

Have a new 16" MBPro and migrating from my previous 2 1/2 year old MBP. The previous Mac developed a dangerous issue with the batteries swelling up inside the case so managed to get a Time Machine backup but can't risk switching the old MBP back on as a fire / explosion risk. The old machine was healthy except the battery issue.

Started the Migration around 3 hours ago, for the last 2 hours stuck on transferring documents at 1 hour 56 mins to go. The overall progress bar is at around 30%. Transfer speed started at around 60 Mbs and has steadily dropped to 5 Mbs. The hard drive with the Time Machine back up is a new WD 2 TB USB 3 which was in perfect working order and I am using a brand new out of the box official Apple USB A to USBC adapter. Also the WD drive is spinning and the status light is flashing.

Both old and new Macs have 1TB SDD's and was around 400Gb used on the old Mac with around 50Gb of docs and and maybe 60Gb of emails.

The failure of the old Mac and its dangerous condition was a complete surprise and as I had work deadlines had no choice but to buy a new mac as could not wait for a repair.

Any ideas how long I should give this? As mentioned before can't risk switching the old one back on and the bottom of the case no longer fits as the batteries are too puffed up.