Replace ISEMPTY with ISBLANK

Your formula should be as follows:-

=SUM(FILTER(A:A, ISBLANK(B:B)))

Now you will get the result as 40

Short answer

Instead of using IMPORTRANGE use Google Apps Script

Explanation

According to Change a spreadsheet's locale, time zone, recalculation, and language the recalculation interval for IMPORTRANGE is 30 mins. If shorter recalculation interval is needed then somehow the formula parameters should be changed.

Unfortunately, in the new Google Sheets is not possible to use NOW, RAND or RANDBETWEEN functions, so the alternative is to use a Google Apps Script.

Explanation about the use of non-deterministic functions

In the thread provided in the comment by Jacob Jan Tuinstra it's mentioned the use of an extra URL parameter that it's updated at certain intervals. That thread is from 2011.

The core idea was to add to the URL something like "&workaround="&NOW() and set the recalculation setting to on change and every minute but this could be too frequent as the import task could take more than one minute, so instead of NOW() you should round it to a good enough interval, let's say 10 minutes, then instead of NOW() use

INT(NOW()/TIME(0;10;0))

Let name this formulas time_interval.

The cell formula containing IMPORTRANGE will look like this

=IMPORTRANGE(
  "https://docs.google.com/spreadsheets/d/spredsheet_key&workaround="&time_interval,
  "Sheet1!A1:AB"
 )

Unfortunately the above formula returns the following error

Error
This function is not allowed to reference a cell with NOW(), RAND(), or RANDBETWEEN()

The same happens with the other import functions (IMPORTDATA, IMPORTHTML, IMPORTFEED, IMPORTXML)

References

• Extend Google Docs, Sheets, and Forms with Apps Script