This fact is probably not documented very well but this is an important factor in SSRS visual report design…
The use of rectangles is very much a core part of Reporting Services report design and a difference in behavior is expected. Cells in a tablix will grow vertically to accommodate wrapping text and when that happens any object on the same row will stretch to fit the row height. For images, charts (which are rendered as images), gauges, indicators (which are gauges) and sparklines (which are charts); this means that the image must be resized and scaled to fit. Image scaling adds a lot of rendering overhead. If you place any of these objects in a rectangle, this prevents the image scaling and speeds things up considerably. This is by design and a recommended practice.
Melissa Coates posted a related topic:
In several comments to that post, users noted slow rendering when charts and sparklines were used in a table. In addition to the rectangle trick, one user suggested “Setting the SizeToReportContent to false on the report viewer allows the images to be consolidated into one image and the performance improves dramatically.”
In July, 2012, Jason Thomas posted on a similar topic:
In that post, he provides step-by-step instructions with images, to use rectangles in a dashboard-style report to correct the rendering issue. He also has several posts on related topics.