The L1 line of RIS references and possible more, when importing a reference, make the program go to the network to check the link to the cited article. With a slow network connection this can mele the import take several minutes or can even make the program spin.
My suggestion is to make use of the L1 line optional on installation of the program. Not having a direct URL access to the referenced article may slow subsequent access to that article but there is no other loss if L1 is not included.
EndNote technical support already provided me with an import filter that doesn’t look for L1 and this made my imports instantenous instead of several minutes.