So https://en.wikipedia.org/wiki/MCST is a SPARC but not Sun Microsystems https://en.wikipedia.org/wiki/SPARC ? Didn't find it in the https://en.wikipedia.org/wiki/List_of_Linux-supported_computer_architectures. So MCST has an own team of Linux kernel developers porting the OS to Elbrus?
On the number of simultaneous users, two measurements come up in this forum. The first one is the number of "users for whom the server is actively doing something" at any instant: https://docs.moodle.org/en/Performance_FAQ#How_do_you_define_.22concurrent_users.22.3F, which is a bit too theoretical for the typical system administrator. The more common one is, how many candidates can enter a schedules MC exam at the same time - the "time" is nowhere specified, but depending on the examination setup could be anything between a couple of seconds to couple of minutes.
On clustering, I saw that you've discussed the topic in the Russian forum Установка и настройка Moodle в кластере на платформе Эльбрус and was aware of the documentation https://docs.moodle.org/en/Server_cluster. And your settings are documented in Настройка балансировщика нагрузки в кластере.
Whatever the measurement you used "100 simultaneous users" sound disappointing for a three machine cluster. As Alex has already suggested, have you tried a single machine setup for comparison? What is the https://moodle.org/plugins/report_benchmark result of a single machine configuration? Did you go through the https://docs.moodle.org/en/Performance_recommendations ?