As a user of Salesforce for so many years, I was used to working with Salesforce Classic but with the release of Lightning UI, I am falling in love with the new UI.While there are so many positive things with lightning but I also faced many frustrations with using Lightning UI. Tha majority of these frustrations have to do with the overall user experience which can make Lightning Experience unproductive.
Here are Top 5 User Experience issues in Lightning  UI. Let’s cover them one by on  in this blogpost.
1) The absence of Text wrapping:
One main reason which makes Salesforce classic so productive is the related lists which allowed text wrapping of fields. With up to 10 fields per related list, field truncation would have been easy, but Salesforce classic wrapped text based on the size of the browser window so that the full text is visible which makes selecting a record very straight forward.

2) Time to propagate configuration changes:

If you make configuration changes using Salesforce classic, they are fast and changes are applied in real-time but that’s not the case with Lightning UI. In most cases, I had to refresh a page multiple times to see the configuration updates.

3) Save Report instead of Run report:

Salesforce admins live in reports. The majority of the time we run reports without saving them. Sometimes we need to run reports just for one-time use and only for a specific purpose, never to be used again. In my use of reports, I use them to perform audits or get a total record count for an object.
Lightning experience doesn’t let the users to create these throw away reports without saving them first. So as an admin, I have to remember to go back and delete the report.A simple workaround is to create a Trash folder where we can save the report and then delete it from trash folder but that seems like an extra and unnecessary step.

4) Background Record page loading time:

I use tabs all the time in my Salesforce org’s. At any given time, I have atleast 5 salesforce windows open for the org I am working in. Salesforce classic allows me to open links in a new tab and when I navigate to the page, record page has already loaded.
But in lightning experience, while it is getting better, records don’t open in the background that quickly at all!it takes several seconds to load the record after I have clicked on the tab.

5) Open in a new tab within setup:

When working with Salesforce setup, not all setup menu has been updated to Lightning experience. The salesforce classic UI doesn’t operate the same in Lightning experience. For example, I have to update some profiles through lightning UI.
In Salesforce classic, I would navigate to Profiles, then open each of the profiles I want to edit in a new Chrome tab.  But in lightning experience, If I try to do the same thing, the tab opens up but displays nothing. So either I need to duplicate the profiles tab for each profile which needs to edited and click into the profiles from the new tabs or click into each profile in a single tab to make the updates, then navigate back to the profiles list.