Migration Assistant Taking Long Time for MacBook Pro over Thunderbolt

Move content to a new Mac - Apple support

Transfer to a new Mac with Migration Assistant - Apple Support

Migration Assistant 'takes over' both computers, and takes a surprisingly long elapsed time. First it may need to compute a Spotlight index of the data. Once data transfer begins, it takes a bit longer than a FULL backup, likely all afternoon to overnight. You may want to set this up late in the day and let it run overnight, and be ready for it not to be done by morning.

"the best way" is to use your Time machine backup from the old Mac as the source for Migration Assistant running on the new Mac. Connecting that drive using USB-2 is as fast as almost every Rotating Magnetic drive, and will not produce a noticeable slowdown doing this transfer.

The way that usually works (but occasionally take several tries) but will seem really slow is using Wi-Fi through your Router. Your old Mac is unlikely to be able to attain faster-than-Gigabit Wi-Fi speeds.

If you could use Ethernet through your Router to BOTH Macs, in most cases that would be much faster. OR, if your old Mac is running 10.12 Sierra or later it can establish an Ad-hoc private Wi-fi connection to the new Mac when placed near the new Mac and both running Migration Assistant.

If your old Mac has no Thunderbolt-3 ports, Thunderbolt Bridge (a direct connection between the two Macs with a ThunderBolt cable) is off the table. [Purchasing a ThunderBolt-3 <-> Thunderbolt-2 adapter AND Thunderbolt-2 cable exceeds US$75 for a one-off transfer, and is not recommenced because of that cost.]

A USB cord sounds like a great idea, but does not work because USB is a local peripheral interface, and a Network interface is required, unless you can make your old Mac ‘look like a drive' to the new Mac.

Target Disk Mode can allow your old Mac to become a Hard drive, and it can be cable-connected to the new Mac. It requires ThunderBolt cable connection, so for a Thunderbolt-2 old Mac, you would need to obtain a Thunderbolt-3 <-> ThunderBolt-2 adapter (US$50) and a Thunderbolt-2 cable.

Transfer files between two Mac computers using target disk mode - Apple Support

Transfer files between two Mac computers using target disk mode - Apple Support

if you still have a lot of files on DropBox, all of those will at least need to be sync-ed, and possibly need to be downloaded to your Mac before they can be copied to the backup drive. That could take quite a lot more elapsed time.