It is well known problem, that sometimes importing downloaded headers takes forever. Currently my system is sitting for hours on some 130MB file, while normally header files are considerably smaller and it mostly takes 15-40 seconds (for 1 million headers) to process them. Now, I checked a content of one of such files and headers inside are like this:
1243093807 [PRiVATE] \ce34cfb5f2\::eafc23e40deec7.eba1a60c3d06b6fbf7f8e0c8ae4c8e.cd0b0754::/ab634679c128/ [newzNZB] [1/3] - yEnc (98/1869) 1339535556 nEwZ[NZB] <pr3d@NET.world> Mon, 30 Jan 2017 09:12:48 GMT <XsHkTlMfJmXwPpInRtEeKhQr-newzNZB-1485767568443@PRIVATE> 739480 5681 Xref: number.nntp.giganews.com alt.binaries.moovee:483384344 alt.binaries.sounds:193527395 alt.binaries.erotica:5102233638 alt.binaries.teevee:1243093807 alt.binaries.inner-sanctum:1021956698
So, header file is filled mostly with cross-posted spam. How NB processes it, I obviously don't know, but something in this seems to cause problems.
Actually I would like to get rid of all such cross-posted spam already at download stage, but this is a bit different story.