20% / 45% CPU load every time I refresh my homepage, is it normal?

You need to scale your setup. 4 Wordpress sites it quite a lot for a droplet that size. Try caching your sites to reduce Wordpress rendering the same content. I’ve linked some good solutions below.

• Wordfence Falcon Engine
• WP Super Cache
• W3 Total Cache

Additionally, upgrade to PHP7 for increased performance and enable OPCache in PHP. You may also want to upgrade to Nginx v1.6 stable. You should also remove any unused plugins (not just disabling, delete them). And to top it off, make sure your database is clean and optimized for you system. If you have a LAMP/LEMP stack, move the database to its own server as 4 Wordpress sites + a database on a single 1GB droplet is a bit too much. I’ve linked some DigitalOcean tutorials to get you started on some of these.

• How To Optimize Nginx Configuration
• How To Upgrade to PHP 7 on Ubuntu 14.04

use cache look add install html minfty and use cache bro

http {

        add_header rt-Fastcgi-Cache $upstream_cache_status;
        fastcgi_param HTTP_IF_NONE_MATCH $http_if_none_match;
        fastcgi_param HTTP_IF_MODIFIED_SINCE $http_if_modified_since;

        ##
        # Basic Settings
        ##

        sendfile on;
        tcp_nopush on;
        tcp_nodelay on;
        keepalive_timeout 65;
        types_hash_max_size 2048;

        # server_names_hash_bucket_size 64;
        # server_name_in_redirect off;

        include /etc/nginx/mime.types;

        default_type application/octet-stream;
        ##
        # Logging Settings
        ##

        access_log /var/log/nginx/access.log;
        error_log /var/log/nginx/error.log;

        ##
        # Gzip Settings
        ##

        gzip on;
        gzip_disable "msie6";

        server_tokens off;
        gzip_vary on;
        gzip_proxied any;
        gzip_comp_level 6;
        gzip_buffers 16 8k;
        gzip_http_version 1.1;
        gzip_min_length 256;
        gzip_types text/plain text/css application/json application/x-javascript text/xml application/xml application/xml+rss text/javascript application/ld+json application/vnd.ms-fontobject application/x-font-opentype application/x-font-truetype application/x-font-tt$
        gzip_disable "MSIE [1-6]\.";
        #set client body size to 2M #
        client_max_body_size 20M;
        ##
        # nginx-naxsi config
        ##
        # Uncomment it if you installed nginx-naxsi
        ##
        include /etc/nginx/naxsi_core.rules;

add this in default config

location ~*  \.(jpg|jpeg|png|gif|ico|css|js|svg|woff|pdf|html|htm)$ {
                expires max;

        }
 }