Thanks for using Syncfusion products,
Query 1: the import process just hangs the browser.
We have checked the reported issue with your attached excel file. We are unable to reproduce your reported issue, the excel file is loaded fine. We have created simple sample in Essential studio Vol2 Sp1(22.214.171.124) with your requirement and the same can be downloaded from the below link.
Could you please try the attached sample and get back to us with more information or by reproducing the issue in the above sample, so that we could sort out the cause of the issue and provide you with solution.
Query 2: Is there a recommended file size beyond which I should not try to import spreadsheets?
We have analyzed your query with the attached excel file, we would like to let you know that we have passed model of the excel file from the server to client side by JSON string. It will import the file based on the JSON string’s limit irrespective of the file size and the JSON string is based on the rows, columns and cell formatting only. In your case 50,000 rows and 10 columns is imported successfully, which is demonstrated in the above attached sample.