Nextcloud sync writes

[Kam]

Hello,
I have installed latest release of nextcloud 20.0.2 plugin jail on Truenas 12.0-U1
It is home server, so I don't have any slog here. Normally there are 0 operations on zil.

I was checking statistics of zil and was surprised, that during accessing any nextcloud page and mainly when I start playing video from firefox, zil activity starts:

root@server:~ # zilstat 2
N-Bytes N-Bytes/s N-Max-Rate B-Bytes B-Bytes/s B-Max-Rate ops <=4kB 4-32kB >=32kB
1068864 534432 1067976 4071424 2035712 3940352 40 0 1 39
336976 168488 200024 1835008 917504 1179648 14 0 0 14
31584 15792 29808 1933312 966656 1671168 18 0 2 16
586400 293200 583224 2584576 1292288 2547712 28 0 6 22
10856 5428 10856 696320 348160 696320 10 0 0 10
85808 42904 64048 901120 450560 712704 62 25 31 6
14944 7472 11256 49152 24576 36864 12 12 0 0
30656 15328 17280 184320 92160 135168 17 3 14 0
58448 29224 40648 741376 370688 520192 27 1 9 17
22072 11036 15120 49152 24576 32768 10 9 1 0
60096 30048 34208 421888 210944 249856 47 19 28 0

Wy are there so much sync operations during loading nextcloud page?
And mainly why each video playback in browser like firefox starts to generate too much zil writes? I thought that playback should not need any sync writes, it is just reading media from disk. It takes long time to even start playback, downloading to client computer is faster.
If sync writes are needed, whould slog help here?

Thanks,
Kam