Pelase excuse us for the delayed reply. Can you please confirm whether you are using StyleManager, or the theming mechanism that is based on implicit styles? Generally, the latter one is significantly faster in terms of load time. Additionally, you can cosider using the lightweight templates that we are shipping, if you have not tested them already.
As for your specific request regarding the CreateItem method of GridViewRow, you are right that this won't be a breaking change and it seems logical that a GridViewRow's children (cells) should be open for extension. However, we would want to take some time to test it before taking any further action.
As a side note, please be advised that any drastic customizations that interfere with the core logic of GridViewCell may be harmful for the working behavior of RadGridView.
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