Raspberry + BubbleUPnP: slow library browsing

I'm giving Serviio a try on my Raspberry to stream video to my Chromecast. To start streaming I use BubbleUPnP on Android. Since most of my videos are in a Chromecast-compatible format and I can use BubbleUPnP server on my laptop when required, I require only very few features from Serviio. Basically I should be able to browse my library and subtitles are supported. And while this all works, browsing the library with BubbleUPnP is pretty slow. It depends on the amount of files/folders in a folder, but it easily takes 10 - 20 seconds to open a folder, sometimes even longer. I already disabled most features such as thumbnail-generation, metadata fetching, dvd covers etc. Is there anything I can do to make the library browsing faster? I don't mind disabling features, as long as I can simply play videos with subtitles.

I know that Raspberry has very few CPU power, but I believe this is something specific to Serviio. I also have Mediatomb (another DLNA server) running on my RPi with exactly the same videos and when I connect from BubbleUPnP to Mediatomb most folders open within 1 second. Load on my Raspberry Pi is mostly below 0.50 so it's not that my RPi is doing a lot of other things while I'm browsing the Serviio library. One other thing I noticed: my videos are on an external network disk (Samba mount on the RPi), everytime I open a Serviio-folder in BubbleUPnP I can see and hear activity on that disk, which seems a bit strange as the medialibrary (derby db) is stored on the SDCard of my RPi and i have disabled the DVD cover features etc.

Thanks for any help.

I just installed JDK 1.8.0 on my Raspberry. At first sight this seems to make a very big difference over the default JDK 1.7 which is in the Raspbian repository.