Tracing the SharePoint Online Performance Issue

Recently I  happen to suggest some of the points to the  real time issue on SharePoint Online. An user experienced that his site is extremely slow due to some unknown reasons.

This site has been recently migrated to O365 from on premise SharePoint 2013 server through 3rd party migration tool.

Here are my suggestions to narrow down the issue.

I have asked him to first check the "Service Health" for SharePoint Online. You have to be O365 tenant administrator to access the Service Health details on Admin Center.

There you can find the SharePoint Online health service, If there is any issue on this, Microsoft categories the severity of the problem in to three category as healthy, advisory and incident.

If there is serious service degradation then it will be logged into Incident category and list all the impacted services like shown in this image.

You have to wait till this issue get resolved from Microsoft end.

Fortunately, user's tenant dashboard shows healthy sign, still the SharePoint Online site which is less than 100GB size started to show the slow performance.

There could be below listed causes those are the reason for this performance issues, you can focus on these,

as it is migrated from OnPremise,

1. Please ensure there is no deleted sites/list/libraries on your recycle bin, There could be a chance storage capacity might have been reached the threshold. Scan the Storage metrics on your site collection (_layouts/15/storman.aspx?root=&OrderBy=0&Asc=0&Page=0)
2. Ensure all the folders and file URLs are not exceeding the defined length.
3. If you have Sandbox solutions on your old sites, then try to deactivate unnecessary features on the site collection.
4. Verify the migrated site's master page and layout pages are compatible with O365 site.
5. Finally you can also check the site collection's health (https://tenantname _layouts/15/sitehealthcheck.aspx)

Site Collection Health Check for SharePoint Online will shows all the problems that occurs either before upgrade or post upgrade of you 

site.

So there could be list of incompatibles from your OnPremise site and its artifacts such as

1. Master Page and Layout
2. Content Type Conflicts 
3.  Missing galleries 
4. Missing Site Templates
5. Sandbox Solutions and
6. Unsupported language package references 

You have to resolve all these problems reported by Health Check.

Last but not least,

There could be storage capacity also causes your site to show the performance issue syndrome. You can trace and track of all SharePoint artifacts and its size from "Storage Metric" page (https://tenant.sharepoint.com/_layouts/15/storman.aspx). 

If you short of the storage space , then you can purchase the space from your SharePoint Online administration page for your site collection.