Assuming that's the copy found here on MR, you can chuck the "it's a Linux thing" theory out the window (pun intended), it takes less than 2s over here.
Granted, that is *NOT* with the official Calibre binaries, so, eh, YMMV. (Desktop souped up Skylake, x64, AMD). Also, /tmp lives in RAM (tmpfs).
Code:
Count Page/Word Statistics
do_count_statistics - book_path=/tmp/calibre_4.5.0_tmp_PROEuY/GUM_VG_count_pages/6376.epub, pages_algorithm=0, page_count_mode=Estimate, statistics_to_run=[u'PageCount'], custom_chars_per_page=1500, icu_wordcount=True
do_count_statistics - job started for file book_path=/tmp/calibre_4.5.0_tmp_PROEuY/GUM_VG_count_pages/6376.epub
-------------------------------
Logfile for book ID 6376 (Complete Works)
Method of counting _page_count_mode=Estimate _download_sources=[]
results= {u'PageCount': 4259}
Found 4259 pages
6376
do_statistics_for_book: /tmp/calibre_4.5.0_tmp_PROEuY/GUM_VG_count_pages/6376.epub 0 Estimate [] [u'PageCount'] 1500 True
Estimated accurate page count
Lines: 132049 Divs: 773 Paras: 42789
Accurate count: 4259 Fast count: 3122
Page count: 4259