synchronization – No new UpdateTip. Just “Pre-allocating up to position …”

I’m IBD’ing a new testnet node 0.21.0

Why are there long periods of no UpdateTip messages? I have 10 connected peers.

Excerpt starting at 00:13:

2021-04-05T00:13:17Z UpdateTip: new best=000000000000005b0a40663b50cf4588f8dfd9c18bbbe8696fe2b549c893fbe4 height=1382700 ...

Then there are lots of pre-alloc messages and no UpdateTip messages:

2021-04-05T00:36:32Z ping timeout: 1200.048936s
2021-04-05T00:36:55Z New outbound peer connected: version: 70015, blocks=1969990, peer=737 (full-relay)
2021-04-05T00:39:17Z Pre-allocating up to position 0x7000000 in blk00112.dat
2021-04-05T00:43:15Z Pre-allocating up to position 0x8000000 in blk00112.dat
2021-04-05T00:44:59Z Socks5() connect to 165.231.163.24:18333 failed: general failure
2021-04-05T00:46:22Z Leaving block file 112: CBlockFileInfo(blocks=167, size=133222507, heights=1382791...1383090, time=2018-08-10...2018-08-11)
2021-04-05T00:46:22Z Pre-allocating up to position 0x1000000 in blk00113.dat
2021-04-05T00:47:43Z New outbound peer connected: version: 70016, blocks=1969991, peer=738 (full-relay)
2021-04-05T00:47:46Z Synchronizing blockheaders, height: 1969991 (~100.00%)
2021-04-05T00:49:10Z Pre-allocating up to position 0x2000000 in blk00113.dat
2021-04-05T00:50:18Z Socks5() connect to 2001:19f0:6001:3bf2:5400:2ff:febc:2f63:18333 failed: general failure
2021-04-05T00:51:03Z Pre-allocating up to position 0x3000000 in blk00113.dat
2021-04-05T00:51:43Z ping timeout: 1200.026721s
2021-04-05T00:51:44Z ping timeout: 1200.029607s
2021-04-05T00:51:57Z New outbound peer connected: version: 70015, blocks=1969991, peer=740 (block-relay)
2021-04-05T00:52:50Z Pre-allocating up to position 0x100000 in rev00113.dat

After about 40 minutes (why?), there is a new UpdateTip finally:

2021-04-05T00:52:50Z UpdateTip: new best=00000000000000c771e3ffe2108fe02768164469d366e2f48c9e8b4ef7eeae54 height=1382701 version=0x20000000 log2_work=71.161934 tx=28150831 date='2018-08-10T05:40:01Z' progress=0.470404 cache=2824.5MiB(20668057txo)

2021-04-05T00:52:50Z UpdateTip: new best=00000000000000ad0e7806e25ae3628c709cbfd1eb100e8b4aeebf326fc603d9 height=1382702 version=0x20000000 log2_work=71.161974 tx=28152664 date='2018-08-10T05:44:22Z' progress=0.470435 cache=2824.6MiB(20668080txo)

I inspected machine health: Very little io-wait, and cpu is quite idle:

procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 2  0 1523548 477268 133600 5865132    0    0    18    29    4    3  6  0 93  1  0
 4  0 1523548 477836 133600 5866108    0    0     0   580 3894 7616  2  1 91  6  0
 1  2 1523548 477648 133600 5866140    0    0    32   400 3650 6994  2  1 88  9  0
 0  1 1523548 477420 133600 5866140    0    0     0   452 4403 8249  1  2 91  6  0
 1  2 1523548 477696 133600 5866140    0    0     0   700 4414 8394  1  1 92  6  0
 0  1 1523548 477004 133616 5866140    0    0     0  1512 3007 4780  1  1 80 19  0
 2  1 1523548 476420 133616 5866144    0    0     0   460 4536 8690  2  2 92  5  0
 0  1 1523548 475532 133616 5866220    0    0     0   480 4616 9066  1  2 93  4  0
 2  1 1523548 475428 133616 5866220    0    0     0   480 4568 8689  1  2 92  5  0

Quad-core, Intel i7-3740QM CPU @ 2.70GHz. I am using conventional hard disk (platters).

I’m curious what might cause the delay. Reviewed the log and IBD seemed to progress steadily until this time.
I searched here and google but could not find an answer.