I would say monitor the fragmentation and keep doing reorg.

Then look at the rebuild options, in particular parallelism.

Also when you run rebuild, make sure your data files and log files are pre-grown to handle the work.

It is possible that the performance hit you got when running the rebuild was due to stalls related to auto-grow of both the data file and log file.

You probably need free space in both the log file and data file about 1.5 times the size of the bigger of the two tables.

Rebuild indexes on one table. Dump the log and make sure it is empty. Then rebuild indexes of the second table