Each Composer block is a page type in EPiServer. When your editors drag a block from the toolbox unto their page a page is created based on that block's page type. Render time of the block depends on what it does, always good to do performance analyzing on the code and cache data that is used regularly.
Hope this helps.
Yes, it's correct that each composer block is using a page (pagedata) for storing the data and other information, but the block itselft is an usercontrol and the rendering time depends on what it does. The block data (pagedata) has a cache-handling the same way as a standard cms page. If you have several blocks of the same composer function type then you have difference instances of block data of a composer page type ( equivalent to several pages of a page type). However, there is a global block which is a single instance of data for all blocks, cross pages on the site.
Note: In the current version (Composer 4 R2), Composer doesn't support the partital cache for the Composer block on page.
Thank you for clarifying this.
I have a couple of questions regarding Composer blocks.
Is it so that each composer block on a composer page actually is rendered as a page? (I.e. one composer page with 10 composer blocks actually render as if it's 10 pages?)
If so, what if I have duplicate (or more) blocks of the same composer function on a page; is the loading time of the block in any way more light-weight as the block is already in use on the composer page?