We get progresss reports of how many revisions/second are processed for xml stubs. I looked at some samples: in Jan 2019 and Mar 20 2019 these numbers ranged from 7000 to 22000 revs/sec. During this run (before encountering exceptions which broke the run) the numbers ranged from 450 to 500 revs/sec.
Excerpts:
Jan 1 2019 run enwiki, samples from earlier created (not new) pages for stubs: 2019-01-01 12:47:10: enwiki (ID 255257) 127 pages (108.4|1509.3/sec all|curr), 5000 revs (4266.8|11884.4/sec all|curr), ETA 2019-01-03 21:50:04 [max 876291835] 2019-01-01 12:47:10: enwiki (ID 255258) 236 pages (201.9|2823.8/sec all|curr), 5000 revs (4278.0|11965.1/sec all|curr), ETA 2019-01-03 21:41:04 [max 876291835] 2019-01-01 12:47:11: enwiki (ID 255248) 422 pages (325.8|9631.6/sec all|curr), 7000 revs (5404.5|22823.8/sec all|curr), ETA 2019-01-03 09:49:31 [max 876291835] 2019-01-01 12:47:11: enwiki (ID 255249) 427 pages (331.4|5539.3/sec all|curr), 8000 revs (6208.9|12972.7/sec all|curr), ETA 2019-01-03 03:59:25 [max 876291835] 2019-01-01 12:47:11: enwiki (ID 255251) 179 pages (132.3|3987.0/sec all|curr), 7000 revs (5174.3|22273.6/sec all|curr), ETA 2019-01-03 11:49:45 [max 876291835] Mar 20 2019, similar samples 2019-03-20 15:12:20: enwiki (ID 265013) 7878 pages (597.9|139556.8/sec all|curr), 67000 revs (5085.4|17714.8/sec all|curr), ETA 2019-03-22 15:44:33 [max 888653661] 2019-03-20 15:12:20: enwiki (ID 265030) 6679 pages (537.9|52339.6/sec all|curr), 55000 revs (4429.2|7836.4/sec all|curr), ETA 2019-03-22 22:56:01 [max 888653664] 2019-03-20 15:12:20: enwiki (ID 264996) 11100 pages (840.6|174888.7/sec all|curr), 96000 revs (7269.9|15755.7/sec all|curr), ETA 2019-03-22 01:09:24 [max 888653661] 2019-03-20 15:12:20: enwiki (ID 264969) 9437 pages (706.8|114008.0/sec all|curr), 100000 revs (7489.5|12081.0/sec all|curr), ETA 2019-03-22 00:09:39 [max 888653661] 2019-03-20 15:12:20: enwiki (ID 264948) 11055 pages (825.4|179958.2/sec all|curr), 127000 revs (9481.8|16278.4/sec all|curr), ETA 2019-03-21 17:14:08 [max 888653661] Apr 1 2019 similar samples 2019-04-01 12:49:10: enwiki (ID 91823) 118 pages (0.7|55.4/sec all|curr), 56000 revs (309.3|469.8/sec all|curr), ETA 2019-05-04 20:32:01 [max 890452225] 2019-04-01 12:49:10: enwiki (ID 91825) 333 pages (1.8|167.8/sec all|curr), 56000 revs (309.3|504.0/sec all|curr), ETA 2019-05-04 20:35:51 [max 890452225] 2019-04-01 12:49:11: enwiki (ID 91801) 240 pages (1.3|113.9/sec all|curr), 58000 revs (319.3|474.4/sec all|curr), ETA 2019-05-03 19:29:28 [max 890452225] 2019-04-01 12:49:11: enwiki (ID 91791) 544 pages (3.0|283.3/sec all|curr), 58000 revs (319.1|520.8/sec all|curr), ETA 2019-05-03 19:57:21 [max 890452225] 2019-04-01 12:49:12: enwiki (ID 91784) 33 pages (0.2|15.1/sec all|curr), 56000 revs (307.1|458.3/sec all|curr), ETA 2019-05-05 02:09:33 [max 890452225]
I have checked (by logging all queries) to see if we inadvertently load revision content, and at least normally we don't, so that's ruled out as a cause.