Hi,

Thank you for sharing your issue.
We were unable to reproduce the issue locally with Bio-Formats 5.6.0, Fiji.

We used the following macro snippet to resolve the time taken over 20 iterations

tarray = newArray();
for(i=0; i<20 ; i++) {
		tic = getTime();
		run("Bio-Formats Importer", "open=/Users/bramalingam/Desktop/17900/mozaicDP4.zvi autoscale color_mode=Default rois_import=[ROI manager] view=Hyperstack stack_order=XYCZT");
		toc = getTime();
		timetaken = toc - tic;
		print(timetaken);
		tarray = Array.concat(tarray, timetaken);
		close();
}
Plot.create("Time taken to open files","Iteration","Time Taken(ms)", tarray);

and we did not see a significant difference in the time taken over the multiple iterations as well.
(Please check the resultant plot attached in here)

It would be great if you could please do the following,
1) run the above macro in your Fiji installation (on one of your files which has had issues) and send us a screenshot of the resultant plot

2) Share your macro code.

This would help us troubleshoot the issue in better detail.

Best,
Balaji