WordPress Admin is sometimes slow and sometimes fast

After updating to WordPress 4.6, my wordpress admin has become so slow. However, sometimes, it’s goes back to regular speed too.

I decided to test if it was a plugin issue but soon realized the inconsistencies in the results when testing. Eg. I deactivated all the plugins. Even before activating one plugin at a time, I started testing by clicking on tabs. Sometimes it loads fast, sometimes it loads slow. I tried to click on the same tabs eg post. Sometimes it takes 2 secs to load, at times it takes more than 10 secs.
Since the problem exists even when all plugins are deactivated, does it mean it’s not a plugin issue?

My website is also sometimes fast and sometimes slow but not necessarily the same time as when the wordpress admin is slow or fast. Sometimes it doesn’t even load at all and shoes this error msg

The https://www.curiousnut.com page isn’t working
https://www.curiousnut.com unexpectedly closed the connection.
ERR_CONTENT_LENGTH_MISMATCH

I tried testing on gtmetrix. Sometimes I get a result when testing speed on gtmetrix. Sometimes, it’ll say that it’s taking too long to load.

I haven’t tried switching themes for I fear that the original theme will be chaos when I switch back.

Website is: https://www.curiousnut.com

Please help.