jose

@Szabolcs Yes, we are aware of this problem. Many thanks again for reporting these issues.

@chuy In 12.1, check the result of Information[PacletObject["TypeSystem"]]["Version"]. If you have the paclet update, this should give "12.1.0.1".

@Szabolcs Hi. We released yesterday an automatic paclet update of the Dataset code. This should make your example behave better. Can you please confirm? Thanks.

@CarlLange Thanks @CarlLange.

@Szabolcs Many thanks for this example, @Szabolcs. We are looking into it too, and hope to have a fix very soon. We are improving Dataset in multiple ways, and WL 12.1 has focused on increased interactivity, Grid-like styling capabilities, storage of data in place, copy-paste, and other FrontEnd-related things. Summary boxes and similar typesetting constructs are important to identify and understand expressions, but they should not make the system much slower, of course.

@CarlLange Hi. This is Jose, from Wolfram. We are aware of an unacceptable slowdown in some Dataset expressions, due to a bad dynamic interaction with the summary boxes of some objects. Both the TimeSeries objects of the coronavirus datasets and the InterpolatingFunction objects in the example given before typeset via summary boxes. We are working on a solution and will release it as soon as it is available.