Ok, we got the answer.
Our developers have made test with this plugin: https://wordpress.org/plugins/wp-page-load-stats/
and these are the results of our themes:
90 queries in 3.763 seconds.
Average load time of 1.7319 (71 runs).
34 out of 40 MB (85%) memory used.
Peak memory usage 35 MB.
and for example this are the results of wordpress default “Twenty Sixteen” theme (this theme is very well optimized):
65 queries in 1.574 seconds.
Average load time of 1.7302 (73 runs).
31 out of 40 MB (78%) memory used.
Peak memory usage 31 MB.
Considering that “Twenty Sixteen” theme has very little functionality it gives a little better results than our theme.
Developers also noted that they have used only functions from WordPress Codex.