I use local storage for backups*

There might be multiple reasons causing the delay. HDD performance, number of small files. There should be no such problem with the new backup format, though. If you still want to stay on the legacy and investigate the issue further, I'd recommend opening a ticket with technical support so that they can review the logs.

Is it safe to use the new format with a large amount ~ 2 tb of small small ~1-10mb files in forever incremental backup mode? What if one of the links in my incremental chain is lost, will I be able to restore the data remaining in the remaining newer links, or will all the data in the links newer than the lost one be also lost?

The new format does not support incremental forever, so it is not a good fit for Cloud file backups as it required periodic "Fulls" which require re-uploading all 2TB of data.
For Image and VHD CLoud backups - and for local drives - the new format works great. You would just need a larger NAS or USB drive.
You could run a FULL once a month and incrementals each night.
If you want to keep 90 days worth of backups of 2TB of data on a local drive, you will consume 8TB plus the daily incremental space consumption.
I would recommend at least 10TB of backup drive space, preferrably12TB.
I also recommend compression and encryption. They do not slow down the backups unless you are on a horrendoesly old slow machine.
I would also encourage you to utilize Wasabi or Backblaze to keep a copy of the data using legacy mode incremental forever in the cloud as a local NAS device can break, get stolen (and not encrypting the data is asking for touble), or be destroyed in a disaster.
Alex is correct, the new format is orders of magnitude faster in bringing up the contents of storage to view/restore than legacy mode for large file stores, but takes significantly more space.
Local space is a one-time and relatively cheap.
Hope this helps.
- Steve

I'm expecting forever forward with the new format to come out soon. I mean it. Stay tuned