I have been trying to synchronize my NIS for days and it is progressing very slowly, I have been there 80 days ago in block 1256073.
Something I can do to synchronize it at a higher speed?
You should try from the latest database found from bob.nem.ninja:
https://bob.nem.ninja/nis5_mainnet.h2-1323k.db.zip
You will delete your local database, and replace it with this bootstrapped one. It will save you time.
Also make sure you are not on a old version of the NIS. The latest version is 6.93:
https://bob.nem.ninja/nis-0.6.93.tgz
Some older versions of NIS will not let you get past a certain block.