All my WordPress websites are slow and plugin page is giving error

That would be quite unusual if it were global. I rather suspect other causes in your case.

Unfortunately, you don’t see the error message you are referring to. If you want to post a screenshot, you can upload it here and link to it: https://imgur.com

You are talking about several WordPress websites. Are they all hosted by the same host? Has he possibly changed something? A question to their support could provide clarification.

If this is not the case, it could also be an update of a plugin, if this was installed automatically. Does this apply to all your websites? A theme could also be the cause.

Have a look at the affected websites under Tools > Site Health to see if any anomalies are mentioned there.

I also want to add some clarification: All WordPress sites are entirely self-contained.

As mentioned above, there could be a problem with your sites (a common configuration, perhaps), or there could be a problem with the server your sites are on, but there will never be a “global issue with WordPress.”