To disable hash calculation on the client uncheck Calculate file-hashes on the client: in the Internet tab.
Other than that I would recommend looking for where the bottleneck is first (it is probably an IO bottleneck so e.g. via iotop): Client, server, which disk, which thread or even which file. E.g. could be the bottleneck is adding file entries to the database. The best fix for that would be putting it on a separate NVMe.