Perhaps this is a network problem. I mean that somewhere between you and Yandex there are network nodes with a low transfer rate. I have excellent speed over WebDAV. Try to synchronize using their original software (Yandex.Disk) or use a proxy.
Unfortunately, a couple of years ago, Yandex introduced throttling when using the WebDAV protocol. For every megabyte of data uploaded or downloaded, Yandex Disk introduces a 60-second delay. For example, if you need to upload a 60MB file to your Yandex Disk storage over WebDAV, it’s going to take you an hour, even on the fastest of Internet connections.
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From:
https://www.datahoards.com/yandex-disk-cloud-storage-faq/
https://discourse.devontechnologies.com/t/sync-with-slow-webdav-could-we-just-wait-instead-of-timeout/60862