I would like to know about Multi DB

I have multisite and my multisite front end loading is good but my backend. When I opened an item menu on my backend like add a post page or adding new page ,new product it will take 8~10 sec to open up the page.
I don't think so it is my host problem i hosted SSD VPS on Wordpress hosting provider here is my server spec
4 x 2.80GHz CPU
4 GB RAM
96 GB SSD Space
4 TB 3 TB Bandwidth
cPanel/WHM/Softaculous
500 Mbit Network Out

I have been increase my site memory to 2048 MB, and i have install some of optimize plug in like Hummingbird ,wp-rocket ,Heartbeat-control,wp-optimize etc on my site ...but my backend still slowness.

I have check my database i got 250 table in the database , i think that is the point lets my backend slowness , but i realize that when i created a new sub site it will generated 10~13 tables that mean when i created more sub site my backend will more slowness ?

I know you guys is the multisite expert can you give me a suggestion or advise what is the solution for my case ? Is i need to install multi DB for my single sub site ?

Cheers

Cole