Nginx + php-fpm + nginx cache is growing $request_time for some reason?

Question

Nginx

There is nginx, nginx cache is used. Config:

user  apache;
worker_processes  8;

error_log  /var/log/nginx/error.log warn;
pid        /var/run/nginx.pid;

worker_rlimit_nofile 65536;

events {
    use epoll;
    multi_accept on;
    worker_connections 2048;
}


http {
    server_names_hash_max_size 512;
    server_names_hash_bucket_size 64;

    include       /etc/nginx/mime.types;
    default_type  application/octet-stream;

    log_format main '$remote_addr - $remote_user [$time_local] "$request" '
                      '$status $body_bytes_sent "$http_referer" '
                      '"$http_user_agent" "$http_x_forwarded_for" $upstream_connect_time - $request_time [$upstream_response_time] $upstream_cache_status';

    access_log off;
    error_log /var/log/nginx/error.log crit;

    keepalive_timeout  45;
    
    client_max_body_size  128m;
    client_body_timeout 35;
    reset_timedout_connection on;
    send_timeout 30;
    
    proxy_max_temp_file_size 0;
    fastcgi_max_temp_file_size 0;
    
    sendfile on;
    aio threads;
    tcp_nopush on;
    tcp_nodelay on;
    server_tokens off;

    fastcgi_cache_path /var/run/nginx-cache levels=1:2 keys_zone=FLASHGAMES:720m inactive=12h use_temp_path=off;
    fastcgi_cache_key "$scheme$request_method$host$request_uri";
    fastcgi_cache_lock off;
    fastcgi_ignore_headers Cache-Control Expires Set-Cookie;

    include /etc/nginx/conf.d/*.conf;
  include /etc/nginx/vhosts/*/*.conf;

    
}

There are many small sites on the site, vhost config:

server {
  server_name blabla.ru www.blabla.ru;
  charset off;
  index index.html index.php;
  disable_symlinks if_not_owner from=$root_path;
  include /etc/nginx/vhosts-includes/*.conf;
  include /etc/nginx/vhosts-resources/blabla.ru/*.conf;
  access_log /var/www/httpd-logs/blabla.ru.access.log;
  error_log /var/www/httpd-logs/blabla.error.log notice;
  ssi on;
  set $root_path /var/www/user30/data/www/blabla.ru;
  root $root_path;
  listen 80;
  
  set $skip_cache 0;
  if ($request_method = POST) {
        set $skip_cache 1;
    }   

    if ($query_string != "") {
        set $skip_cache 1;
    }   

    # Don't cache URIs containing the following segments
    if ($request_uri ~* "/wp-admin/|wp-login.php|/xmlrpc.php|wp-.*.php|/feed/|index.php|sitemap(_index)?.xml|/sitemanage") {
        set $skip_cache 1;
    }  

    # Don't use the cache for logged-in users or recent commenters
    if ($http_cookie ~* "comment_author|wordpress_[a-f0-9]+|wp-postpass|wordpress_no_cache|wordpress_logged_in") {
        set $skip_cache 1;
    }
    
  location / {
      try_files $uri $uri/ /index.php?$args;
    location ~ [^/]\.ph(p\d*|tml)$ {
      try_files /does_not_exists @php;
    }
  }
  gzip on;
  gzip_comp_level 3;
  gzip_disable "msie6";
  gzip_types text/plain text/css application/json application/x-javascript text/xml application/xml application/xml+rss text/javascript application/javascript;

  
  location ~* \.(jpg|jpeg|jpe|gif|png|css|bmp|js|ico|ogg|mp3|flv|swf|ttf|woff|woff2)$ {
    expires max;
    add_header Cache-Control "max-age=88000,  public";
    }
  
    location = /robots.txt {
        access_log off;
        log_not_found off;
    }

    location = /xmlrpc.php {
      deny all;
      access_log off;
      log_not_found off;
    }
    
  location @php {
    fastcgi_index index.php;
    fastcgi_pass unix:/var/www/php-fpm/user30.sock;
    fastcgi_split_path_info ^((?U).+\.ph(?:p\d*|tml))(/?.+)$;
    try_files $uri =404;
    include fastcgi_params;
    
    fastcgi_cache FLASHGAMES;
    fastcgi_cache_valid 200 12h;
    fastcgi_cache_bypass $skip_cache;
    fastcgi_no_cache $skip_cache;
  }
  
}

I'm testing the response time using the Yandex tool https://webmaster.yandex.ru/tools/server-response/
Once checked - ~150ms, second, ~150, .. .. then it can suddenly rise to 400, 1000, etc.
In the logs, these requests look like this (last three parameters $request_time [$upstream_response_time] $upstream_cache_status'):

5.45.203.61 - - [02/Dec/2017:08:04:34 +0300] "GET / HTTP/1.1" 200 34158 "-" "Mozilla/5.0 (compatible; YandexBot/3.0; +http://yandex.com/bots)" "-" - - 0.054 [-] HIT
5.45.203.61 - - [02/Dec/2017:08:04:41 +0300] "GET / HTTP/1.1" 200 34158 "-" "Mozilla/5.0 (compatible; YandexBot/3.0; +http://yandex.com/bots)" "-" - - 0.054 [-] HIT
5.45.203.61 - - [02/Dec/2017:08:04:52 +0300] "GET / HTTP/1.1" 200 34158 "-" "Mozilla/5.0 (compatible; YandexBot/3.0; +http://yandex.com/bots)" "-" - - 0.102 [-] HIT
5.45.203.61 - - [02/Dec/2017:08:05:03 +0300] "GET / HTTP/1.1" 200 34158 "-" "Mozilla/5.0 (compatible; YandexBot/3.0; +http://yandex.com/bots)" "-" - - 0.298 [-] HIT
5.45.203.61 - - [02/Dec/2017:08:05:15 +0300] "GET / HTTP/1.1" 200 34158 "-" "Mozilla/5.0 (compatible; YandexBot/3.0; +http://yandex.com/bots)" "-" - - 0.999 [-] HIT

Nothing in the host error logs. In the general enginx error log -

2017/12/02 08:00:21 [warn] 1490#1490: could not build optimal server_names_hash, you should increase either server_names_hash_max_size: 512 or server_names_hash_bucket_size: 64; ignoring server_names_hash_bucket_size

I don’t understand what this could be connected with, and how to debug, understand what the problem is