Thank you for your post.
I'm not familiar with the differences in client-side and server-side tabbing or the slowness you mention, but if you could get me a sample file that reproduces I would be happy to check it out! Sending you a private message to your inbox.
I reproduced this on a Web Direct hosted file of the default Inventory solution included in FileMaker 14.
1) Host the default 'Inventory' starter solution on FMS 14 via Web Direct
2) Go to the 'Product Details | iPad' layout
3) Try tabbing from one field to another, you'll see a delay of ~30-120ms between hitting the tab button and cursor actually going to the next field. The delay you're seeing is the web client reaching out to the FileMaker server waiting for FileMaker server to apply any validation or checks that might be on that field. According to what the FileMaker PM said above, any field that doesn't have validation ("As soon as validation, script triggers, data formatting, conditional formatting are added we have to use server side tabbing.") should revert to Client Side tabbing (i.e. not waiting for a response from the server to advance to the next field). The Item field on the Inventory layout is a simple, text, indexed field with no validation applied to it either in the Define Database dialog window or any other validation options within the inspector. (Or perhaps it is, one of the issues is the lack of any documentation from FileMaker as to what features require server-side tabbing in order to function.)
Unless I'm missing something and there IS an option enabled for that field that would cause the server-side tabbing to take precedence over client-side, this would appear to not be functioning correctly.
One thing that I noticed on my solution was that the mere presence of an object that DID have validation seemed to negate client-side tabbing altogether.
Actually now I can do you one better. Add 3 fields to the Startup layout and go to the layout in Web Direct and try tabbing between the fields. Notice that it's very responsive.
Now add a blank Popover Button to the layout and try tabbing between the fields again. It appears as if the mere presence of a Popover button makes the entire layout and all tabbed objects default to server-side tabbing for some reason.